Hemispherectomy is a surgery that is performed by a neurosurgeon where an unhealthy hemisphere of the brain is disconnected or removed. There are two types of hemispherectomy. Functionalhemispherectomy
refers to when the diseased brain is simply disconnected so that it can
no longer send signals to the rest of the brain and body. Anatomical hemispherectomy
refers to when not only is there disconnection, but also the diseased
brain is physically removed from the skull. This surgery is mostly used
as a treatment for medically intractable epilepsy, which is the term used when anti-seizure medications are unable to control seizures.
History
The first anatomical hemispherectomy was performed and described in 1928 by Walter Dandy. This was done as an attempt to treat glioma, a brain tumor.
The first known anatomical hemispherectomy performed as a treatment for
intractable epilepsy was in 1938 by Kenneth McKenzie, a Canadian
neurosurgeon.
Krynaw, a neurosurgeon from South Africa, was one of the first to
perform and report a case series on hemispherectomies in 1950. He
performed the surgery on pediatric patients with infantile hemiplegia,
specifically as a treatment for their seizures and cognitive impairment. His hemispherectomy technique removed the damaged hemisphere except the thalamus and caudate
structures. Krynaw reported good outcomes overall, although there was
one post-operative death. Specifically, there was an overall theme of
improvement in weakness, spasticity and cognition. Amazingly, ten out of the twelve patients had seizures prior to the operation and none of the patients had seizures afterwards.
Other neurosurgeons began performing hemispherectomies as well,
primarily for the treatment of seizures. For the most part, the
surgeries would go well initially, but there was a general theme of
subsequent deterioration and even death years after the surgery. As a
result of the complication risk and the introduction of new anti-seizure
medications, the popularity of the procedure began to decline in the
1950s.
Oppenheimer and Griffith were one of the first to describe the
potential complications, and they reported their findings in 1966,
describing superficial hemosiderosis, granular ependymitis and obstructive hydrocephalus. They posited a theoretical solution to this problem, a surgery that is now known as a functional hemispherectomy.
Rasmussen was one of the first neurosurgeons to develop and apply a
functional hemispherectomy in practice. He initially made modifications
to the original hemispherectomy by preserving the least epileptogenic
quarter or third of the hemisphere, hoping this would ameliorate the
known complications of the original anatomic hemispherectomy. Although
this modification seemed to solve this issue, patients undergoing the
modified hemispherectomy continued to have seizures, which was
problematic. Therefore, he further modified his surgery to functionally
sever residual portions of the frontal and parieto-occipital lobes.
This surgery, the functional hemispherectomy, has been further modified
over the years by several different neurosurgeons, and to this day
there is not a consensus as to which exact technique should be used.
Hemispherotomy refers to some of the more recently developed approaches
to disconnect the epileptic hemisphere while minimizing brain removal
and the risk for complications.
Nomenclature
There are two main types of hemispherectomy: Anatomical and Functional.
Anatomical hemispherectomy refers to the resection and removal of an entire hemisphere of the brain, which includes all four lobes, with or without the removal of basal ganglia and thalamus.
Functional hemispherectomy refers to surgeries that
disable the function of one hemisphere, while maintaining its blood
supply and without physically removing the entire hemisphere from the
skull.
Functional hemispherectomies are performed more frequently than
anatomical hemispherectomies due to their lower complication rates.
However, they do carry a risk of incomplete disconnection, which refers
to when the surgeon inadvertently leaves remnants of fibers that
continue to connect the hemisphere to the brain and body. These
remaining fibers can be problematic, as they may lead to seizure
recurrence.
Another term that falls under the hemispherectomy umbrella includes hemidecortication,
which is the removal of the cortex from one half of the cerebrum, while
attempting to preserve the ventricular system by maintaining the
surrounding white matter. Hemidecortication was originally developed as a
possible strategy to mitigate some of the complications seen with
complete anatomical hemispherectomy.
The term hemispherotomy refers to a surgery that is akin
to a functional hemispherectomy in that it functionally severs the
damaged hemisphere from the other and leaves some of the severed
hemisphere within the skull, but the difference is that it removes even
less tissue from the skull.
The term hemispherotomy is now used as an umbrella term to describe the
group of modern techniques and procedures that predominate at most
contemporary epilepsy centers.
There is no statistically significant difference in seizure-free
rates between the four different types of surgeries: Hemispherotomy,
functional hemispherectomy, anatomical hemispherectomy and
hemidecortication. The overall rate of seizure freedom is estimated to
be 73.4%. However, hemispherotomy procedures may be associated with a more favorable complication profile.
Candidates
The
typical candidates for hemispherectomy are pediatric patients who have
intractable epilepsy due to extensive cerebral unilateral hemispheric
injuries.
In addition, the seizures should ideally be emanating from that same
hemisphere. In some situations, a hemispherectomy may still be performed
if there are seizures from both hemispheres, as long as the majority
come from one side. In order to assess the patient’s epilepsy
completely, patients undergo extensive testing, including EEG and MRI. Most patients also undergo other studies including functional MRI (fMRI), positron emission tomography (PET) or magnetoencephalography (MEG).
Today, hemispherectomy is performed as a treatment for severe and
intractable epilepsy, including for young children whose epilepsy has
been found to be drug-resistant. The most common underlying etiologies include malformations of cortical development (MCD), perinatal stroke and Rasmussen’s encephalitis. MCD is an umbrella term for a wide variety of developmental brain anomalies, including hemimegalencephaly and cortical dysplasia. Other less common underlying etiologies include hemiconvulsion-hemiplegia epilepsy syndrome and Sturge-Weber syndrome.
Procedure
Patients often shave the area of the scalp that will be involved with the surgery. Patients undergo general anesthesia
and are unconscious for the procedure. The surgical site is sterilized,
after which the skin is incised. A substantial portion of the bone is
removed, followed by incision of the dura,
which is the outer covering of the brain. There are several blood
vessels that have connections with both sides of the brain, and these
are carefully identified and clipped in such a way that spares the
healthy hemisphere. Ultimately, a bundle of fibers that connect both of
the cerebral hemispheres, the corpus callosum,
is removed which results in the functional separation of one hemisphere
from the other. Portions of the cerebral lobes from the damaged side of
the brain are removed, depending on the specific procedure being
performed. The surgeon may leave some brain tissue, such as the thalamus or choroid plexus.
After completing the resection, the surgical site is irrigated with
saline, the brain covering called the dura is sutured back together, the
bone that was removed is replaced and the skin is sutured. This surgery
often takes four to five hours.
Patients often spend a few nights in the hospital post-operatively, and
they undergo physical and occupational therapy soon after the surgery.
Potential complications
The most common complication from surgery is hydrocephalus,
a condition in which fluid accumulates within the brain, and this is
often treated with a shunt to divert the fluid away. The rate of shunts
following surgery ranges from 14–23%. Other complications include wound complications, epidural hemorrhages, subdural hemorrhages, intraparenchymal hemorrhages, intracranial abscesses, meningitis, ventriculitis and venous thrombosis. Additional epilepsy surgery following hemispherectomy is rare (4.5%),[7]
but may be recommended if there is a residual connection between the
two hemispheres that is causing frequent seizures. Mortality rates are
low and estimated to be <1% to 2.2%. Most patients do not experience changes in cognition, but some individuals may be at risk.
A visual deficit called contralateral homonymous hemianopsia is
expected to occur in most patients, where the entire visual field
contralateral to the removed hemisphere is lost. There is a risk of motor deficits, and this is variable. Other possible complications include infection, aseptic meningitis, hearing loss, endocrine problems and transient neurologic deficits such as limb weakness.
Outcomes
Since
seizures are the most common indication for hemispherectomy surgery,
most research on hemispherectomy analyzes how the surgery affects
seizures. Many patients undergoing surgery obtain good surgical
outcomes, some obtaining complete seizure freedom (54–90%) and others
having some degree of improvement in seizure burden.
A recently developed scoring system has been proposed to help predict
the probability of seizure freedom with more accuracy: HOPS
(Hemispherectomy Outcome Prediction Scale).
Although it cannot definitively predict surgical outcome with exact
precision, some physicians may use it as a guide. The scoring system
takes certain variables into consideration including age at seizure
onset, history of prior brain surgery, seizure semiology and imaging
findings.
There is also data pertaining to how hemispherectomy affects the
body in other ways. After surgery, the remaining cerebral hemisphere is
often able to take over some cognitive, sensory and motor functions. The
degree to which the remaining hemisphere takes on this additional
workload often depends on several factors, including the underlying
etiology, which hemisphere is removed and the age at which the surgery
occurs.
In terms of postoperative motor function, some patients may have improvement or no change of their weaker extremity, and many can walk independently. Most patients postoperatively have minimal to no behavioral problems, satisfactory language skills, good reading capability, and only a minority of patients have a decline in IQ. Predictors of poor outcome may include seizure recurrence and structural abnormalities in the intact hemisphere.
Ultimately, risks and benefits should be weighed on an individual
basis and discussed in detail with the neurosurgeon. Many patients have
excellent outcomes, and the International League Against Epilepsy
(ILAE) reports that “about one-fifth of hemispherectomy patients are
gainfully employed and even fewer live independently.”
The Brain Recovery Project
The Brain Recovery Project is a non-profit corporation which funds new research and is based in the United States.
This corporation hosts an annual two-day conference for patients who
have had hemispherectomies and their families. There are several
purposes to this reunion. The main goal is to educate patients and their
families on the surgery and its necessary subsequent rehabilitation. It
also serves as a way for patients and families to connect with one
another, learn from specialists in the field and often offers research
enrollment.
The circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate.It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and from Latin vascula meaning vessels). The circulatory system has two divisions, a systemic circulation or circuit, and a pulmonary circulation or circuit. Some sources use the terms cardiovascular system and vascular system interchangeably with circulatory system.
In vertebrates, the lymphatic system is complementary to the circulatory system. The lymphatic system carries excess plasma (filtered from the circulatory system capillaries as interstitial fluid between cells) away from the body tissues via accessory routes that return excess fluid back to blood circulation as lymph.
The lymphatic system is a subsystem that is essential for the
functioning of the blood circulatory system; without it the blood would
become depleted of fluid.
The lymphatic system also works with the immune system. The circulation of lymph takes much longer than that of blood and, unlike the closed (blood) circulatory system, the lymphatic system is an open system. Some sources describe it as a secondary circulatory system.
The circulatory system can be affected by many cardiovascular diseases. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on disorders of the blood vessels, and lymphatic vessels.
Structure
Blood flow in the pulmonary and systemic circulations showing capillary networks in the torso sections
The circulatory system includes the heart, blood vessels, and blood. The cardiovascular system
in all vertebrates, consists of the heart and blood vessels. The
circulatory system is further divided into two major circuits – a pulmonary circulation, and a systemic circulation. The pulmonary circulation is a circuit loop from the right heart taking deoxygenated blood to the lungs where it is oxygenated and returned to the left heart.
The systemic circulation is a circuit loop that delivers oxygenated
blood from the left heart to the rest of the body, and returns
deoxygenated blood back to the right heart via large veins known as the venae cavae. The systemic circulation can also be defined as two parts – a macrocirculation and a microcirculation.
An average adult contains five to six quarts (roughly 4.7 to 5.7
liters) of blood, accounting for approximately 7% of their total body
weight. Blood consists of plasma, red blood cells, white blood cells, and platelets. The digestive system also works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.
Diagram of the human heart showing blood oxygenation to the pulmonary and systemic circulation
The heart pumps blood to all parts of the body providing nutrients and oxygen to every cell, and removing waste products. The left heart pumps oxygenated blood returned from the lungs to the rest of the body in the systemic circulation. The right heart pumps deoxygenated blood to the lungs in the pulmonary circulation. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle.
The right atrium is the upper chamber of the right side of the heart.
The blood that is returned to the right atrium is deoxygenated (poor in
oxygen) and passed into the right ventricle to be pumped through the
pulmonary artery to the lungs for re-oxygenation and removal of carbon
dioxide. The left atrium receives newly oxygenated blood from the lungs
as well as the pulmonary vein which is passed into the strong left
ventricle to be pumped through the aorta to the different organs of the
body.
The pulmonary circulation is the part of the circulatory system in which oxygen-depleted blood is pumped away from the heart, via the pulmonary artery, to the lungs and returned, oxygenated, to the heart via the pulmonary vein.
Oxygen-deprived blood from the superior and inferior vena cava enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the left atrium.
A separate circuit from the systemic circulation, the bronchial circulation supplies blood to the tissue of the larger airways of the lung.
Systemic circulation
Capillary bedDiagram of capillary network joining the arterial system with the venous system
The systemic circulation is a circuit loop that delivers oxygenated
blood from the left heart to the rest of the body through the aorta. Deoxygenated blood is returned in the systemic circulation to the right heart via two large veins, the inferior vena cava and superior vena cava,
where it is pumped from the right atrium into the pulmonary circulation
for oxygenation. The systemic circulation can also be defined as having
two parts – a macrocirculation and a microcirculation.
Depiction of the heart, major veins and arteries constructed from body scans
Oxygenated blood enters the systemic circulation when leaving the left ventricle, via the aortic semilunar valve.
The first part of the systemic circulation is the aorta, a massive and
thick-walled artery. The aorta arches and gives branches supplying the
upper part of the body after passing through the aortic opening of the
diaphragm at the level of thoracic ten vertebra, it enters the abdomen. Later, it descends down and supplies branches to abdomen, pelvis, perineum and the lower limbs.
The walls of the aorta are elastic. This elasticity helps to maintain the blood pressure throughout the body.
When the aorta receives almost five litres of blood from the heart, it
recoils and is responsible for pulsating blood pressure. As the aorta
branches into smaller arteries, their elasticity goes on decreasing and
their compliance goes on increasing.
Capillaries
Arteries branch into small passages called arterioles and then into the capillaries. The capillaries merge to bring blood into the venous system. The total length of muscle capillaries in a 70 kg human is estimated to be between 9,000 and 19,000 km.
Capillaries merge into venules, which merge into veins. The venous system
feeds into the two major veins: the superior vena cava – which mainly
drains tissues above the heart – and the inferior vena cava – which
mainly drains tissues below the heart. These two large veins empty into
the right atrium of the heart.
The general rule is that arteries from the heart branch out into
capillaries, which collect into veins leading back to the heart. Portal veins are a slight exception to this. In humans, the only significant example is the hepatic portal vein which combines from capillaries around the gastrointestinal tract
where the blood absorbs the various products of digestion; rather than
leading directly back to the heart, the hepatic portal vein branches
into a second capillary system in the liver.
The heart itself is supplied with oxygen and nutrients through a
small "loop" of the systemic circulation and derives very little from
the blood contained within the four chambers.
The coronary circulation system provides a blood supply to the heart muscle itself. The coronary circulation begins near the origin of the aorta by two coronary arteries: the right coronary artery and the left coronary artery. After nourishing the heart muscle, blood returns through the coronary veins into the coronary sinus and from this one into the right atrium. Backflow of blood through its opening during atrial systole is prevented by the Thebesian valve. The smallest cardiac veins drain directly into the heart chambers.
The brain has a dual blood supply, an anterior and a posterior circulation from arteries at its front and back. The anterior circulation arises from the internal carotid arteries to supply the front of the brain. The posterior circulation arises from the vertebral arteries, to supply the back of the brain and brainstem. The circulation from the front and the back join (anastomise) at the circle of Willis. The neurovascular unit,
composed of various cells and vasculature channels within the brain,
regulates the flow of blood to activated neurons in order to satisfy
their high energy demands.
Renal circulation
The renal circulation is the blood supply to the kidneys, contains many specialized blood vessels and receives around 20% of the cardiac output. It branches from the abdominal aorta and returns blood to the ascending inferior vena cava.
The development of the circulatory system starts with vasculogenesis in the embryo.
The human arterial and venous systems develop from different areas in
the embryo. The arterial system develops mainly from the aortic arches,
six pairs of arches that develop on the upper part of the embryo. The
venous system arises from three bilateral veins during weeks 4 – 8 of embryogenesis. Fetal circulation begins within the 8th week of development. Fetal circulation does not include the lungs, which are bypassed via the truncus arteriosus. Before birth the fetus obtains oxygen (and nutrients) from the mother through the placenta and the umbilical cord.
Animation
of a typical human red blood cell cycle in the circulatory system. This
animation occurs at a faster rate (~20 seconds of the average 60-second cycle)
and shows the red blood cell deforming as it enters capillaries, as
well as the bars changing color as the cell alternates in states of
oxygenation along the circulatory system.
The human arterial system originates from the aortic arches and from the dorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and form only the maxillary arteries and stapedial arteries respectively. The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses).
The dorsal aortae, present on the dorsal
side of the embryo, are initially present on both sides of the embryo.
They later fuse to form the basis for the aorta itself. Approximately
thirty smaller arteries branch from this at the back and sides. These
branches form the intercostal arteries,
arteries of the arms and legs, lumbar arteries and the lateral sacral
arteries. Branches to the sides of the aorta will form the definitive renal, suprarenal and gonadal arteries. Finally, branches at the front of the aorta consist of the vitelline arteries and umbilical arteries. The vitelline arteries form the celiac, superior and inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.
About 98.5% of the oxygen in a sample of arterial blood in a healthy human, breathing air at sea-level pressure, is chemically combined with hemoglobin
molecules. About 1.5% is physically dissolved in the other blood
liquids and not connected to hemoglobin. The hemoglobin molecule is the
primary transporter of oxygen in vertebrates.
Diseases affecting the cardiovascular system are called cardiovascular disease.
Many of these diseases are called "lifestyle diseases"
because they develop over time and are related to a person's exercise
habits, diet, whether they smoke, and other lifestyle choices a person
makes. Atherosclerosis is the precursor to many of these diseases. It is where small atheromatous plaques
build up in the walls of medium and large arteries. This may eventually
grow or rupture to occlude the arteries. It is also a risk factor for acute coronary syndromes,
which are diseases that are characterised by a sudden deficit of
oxygenated blood to the heart tissue. Atherosclerosis is also associated
with problems such as aneurysm formation or splitting ("dissection") of arteries.
Another major cardiovascular disease involves the creation of a clot, called a "thrombus". These can originate in veins or arteries. Deep venous thrombosis,
which mostly occurs in the legs, is one cause of clots in the veins of
the legs, particularly when a person has been stationary for a long
time. These clots may embolise, meaning travel to another location in the body. The results of this may include pulmonary embolus, transient ischaemic attacks, or stroke.
Cardiovascular diseases may also be congenital in nature, such as heart defects or persistent fetal circulation,
where the circulatory changes that are supposed to happen after birth
do not. Not all congenital changes to the circulatory system are
associated with diseases, a large number are anatomical variations.
The function and health of the circulatory system and its parts are
measured in a variety of manual and automated ways. These include simple
methods such as those that are part of the cardiovascular examination, including the taking of a person's pulse as an indicator of a person's heart rate, the taking of blood pressure through a sphygmomanometer or the use of a stethoscope to listen to the heart for murmurs which may indicate problems with the heart's valves. An electrocardiogram can also be used to evaluate the way in which electricity is conducted through the heart.
Cardiovascular procedures are more likely to be performed in an
inpatient setting than in an ambulatory care setting; in the United
States, only 28% of cardiovascular surgeries were performed in the
ambulatory care setting.
Other animals
The
open circulatory system of the grasshopper – made up of a heart,
vessels and hemolymph. The hemolymph is pumped through the heart, into
the aorta, dispersed into the head and throughout the hemocoel, then
back through the ostia in the heart and the process repeated.
While humans, as well as other vertebrates,
have a closed blood circulatory system (meaning that the blood never
leaves the network of arteries, veins and capillaries), some invertebrate groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, diploblastic animal phyla lack circulatory systems.
An additional transport system, the lymphatic system, which is
only found in animals with a closed blood circulation, is an open
system providing an accessory route for excess interstitial fluid to be
returned to the blood.
The blood vascular system first appeared probably in an ancestor of the triploblasts over 600 million years ago, overcoming the time-distance constraints of diffusion, while endothelium evolved in an ancestral vertebrate some 540–510 million years ago.
In arthropods, the open circulatory system is a system in which a fluid in a cavity called the hemocoel
bathes the organs directly with oxygen and nutrients, with there being
no distinction between blood and interstitial fluid; this combined fluid
is called hemolymph or haemolymph. Muscular movements by the animal during locomotion
can facilitate hemolymph movement, but diverting flow from one area to
another is limited. When the heart relaxes, blood is drawn back toward
the heart through open-ended pores (ostia).
There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.
Flatworms, such as this Pseudoceros bifurcus, lack specialized circulatory organs.
Closed circulatory system
Two-chambered heart of a fish
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids, octopuses and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as closed, just as in humans. Still, the systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system. Closed systems permit blood to be directed to the organs that require it.
In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers).
In amphibians and most reptiles, a double circulatory system is
used, but the heart is not always completely separated into two pumps.
Amphibians have a three-chambered heart.
In reptiles, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter muscle.
This allows a second possible route of blood flow. Instead of blood
flowing through the pulmonary artery to the lungs, the sphincter may be
contracted to divert this blood flow through the incomplete ventricular
septum into the left ventricle and out through the aorta.
This means the blood flows from the capillaries to the heart and back
to the capillaries instead of to the lungs. This process is useful to ectothermic (cold-blooded) animals in the regulation of their body temperature.
Mammals, birds and crocodilians
show complete separation of the heart into two pumps, for a total of
four heart chambers; it is thought that the four-chambered heart of
birds and crocodilians evolved independently from that of mammals.
Double circulatory systems permit blood to be repressurized after
returning from the lungs, speeding up delivery of oxygen to tissues.
No circulatory system
Circulatory systems are absent in some animals, including flatworms. Their body cavity has no lining or enclosed fluid. Instead, a muscular pharynx leads to an extensively branched digestive system that facilitates direct diffusion
of nutrients to all cells. The flatworm's dorso-ventrally flattened
body shape also restricts the distance of any cell from the digestive
system or the exterior of the organism. Oxygen
can diffuse from the surrounding water into the cells, and carbon
dioxide can diffuse out. Consequently, every cell is able to obtain
nutrients, water and oxygen without the need of a transport system.
Some animals, such as jellyfish, have more extensive branching from their gastrovascular cavity
(which functions as both a place of digestion and a form of
circulation), this branching allows for bodily fluids to reach the outer
layers, since the digestion begins in the inner layers.
History
Human
anatomical chart of blood vessels, with heart, lungs, liver and kidneys
included. Other organs are numbered and arranged around it. Before
cutting out the figures on this page, Vesalius
suggests that readers glue the page onto parchment and gives
instructions on how to assemble the pieces and paste the multilayered
figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD
Collection, WZ 240 V575dhZ 1543.
The earliest known writings on the circulatory system are found in the Ebers Papyrus (16th century BCE), an ancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual. In the papyrus,
it acknowledges the connection of the heart to the arteries. The
Egyptians thought air came in through the mouth and into the lungs and
heart. From the heart, the air travelled to every member through the
arteries. Although this concept of the circulatory system is only
partially correct, it represents one of the earliest accounts of
scientific thought.
In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to the Ayurvedic physician Sushruta in ancient India. He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007). The first major ancient Greek research into the circulatory system was completed by Plato in theTimaeus,
who argues that blood circulates around the body in accordance with the
general rules that govern the motions of the elements in the body;
accordingly, he does not place much importance in the heart itself. The valves of the heart were discovered by a physician of the Hippocratic school around the early 3rd century BC.
However, their function was not properly understood then. Because blood
pools in the veins after death, arteries look empty. Ancient anatomists
assumed they were filled with air and that they were for the transport
of air.
The Greek physician, Herophilus, distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Greek anatomist Erasistratus
observed that arteries that were cut during life bleed. He ascribed the
fact to the phenomenon that air escaping from an artery is replaced
with blood that enters between veins and arteries by very small vessels.
Thus he apparently postulated capillaries but with reversed flow of
blood.
In 2nd-century AD Rome, the Greek physician Galen
knew that blood vessels carried blood and identified venous (dark red)
and arterial (brighter and thinner) blood, each with distinct and
separate functions. Growth and energy were derived from venous blood
created in the liver from chyle, while arterial blood gave vitality by
containing pneuma (air) and originated in the heart. Blood flowed from
both creating organs to all parts of the body where it was consumed and
there was no return of blood to the heart or liver. The heart did not
pump blood around, the heart's motion sucked blood in during diastole
and the blood moved by the pulsation of the arteries themselves.
Galen believed that the arterial blood was created by venous
blood passing from the left ventricle to the right by passing through
'pores' in the interventricular septum, air passed from the lungs via
the pulmonary artery to the left side of the heart. As the arterial
blood was created 'sooty' vapors were created and passed to the lungs
also via the pulmonary artery to be exhaled.
In 1025, The Canon of Medicine by the Persian physician, Avicenna,
"erroneously accepted the Greek notion regarding the existence of a
hole in the ventricular septum by which the blood traveled between the
ventricles." Despite this, Avicenna "correctly wrote on the cardiac cycles and valvular function", and "had a vision of blood circulation" in his Treatise on Pulse. While also refining Galen's erroneous theory of the pulse, Avicenna
provided the first correct explanation of pulsation: "Every beat of the
pulse comprises two movements and two pauses. Thus, expansion : pause :
contraction : pause. [...] The pulse is a movement in the heart and
arteries ... which takes the form of alternate expansion and
contraction."
In 1242, the Arabian physician, Ibn al-Nafis described the process of pulmonary circulation in greater, more accurate detail than his predecessors, though he believed, as they did, in the notion of vital spirit (pneuma), which he believed was formed in the left ventricle. Ibn al-Nafis stated in his Commentary on Anatomy in Avicenna's Canon:
...the blood from the right chamber of the heart must
arrive at the left chamber but there is no direct pathway between them.
The thick septum of the heart is not perforated and does not have
visible pores as some people thought or invisible pores as Galen
thought. The blood from the right chamber must flow through the vena
arteriosa (pulmonary artery) to the lungs, spread through its
substances, be mingled there with air, pass through the arteria venosa (pulmonary vein) to reach the left chamber of the heart and there form the vital spirit...
In addition, Ibn al-Nafis had an insight into what would become a larger theory of the capillary circulation. He stated that "there must be small communications or pores (manafidh
in Arabic) between the pulmonary artery and vein," a prediction that
preceded the discovery of the capillary system by more than 400 years. Ibn al-Nafis' theory, however, was confined to blood transit in the lungs and did not extend to the entire body.
Michael Servetus
was the first European to describe the function of pulmonary
circulation, although his achievement was not widely recognized at the
time, for a few reasons. He firstly described it in the "Manuscript of
Paris" (near 1546), but this work was never published. And later he published this description, but in a theological treatise, Christianismi Restitutio,
not in a book on medicine. Only three copies of the book survived but
these remained hidden for decades, the rest were burned shortly after
its publication in 1553 because of persecution of Servetus by religious
authorities.
A better known discovery of pulmonary circulation was by Vesalius's successor at Padua, Realdo Colombo, in 1559.
Finally, the English physician William Harvey, a pupil of Hieronymus Fabricius
(who had earlier described the valves of the veins without recognizing
their function), performed a sequence of experiments and published his Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus
in 1628, which "demonstrated that there had to be a direct connection
between the venous and arterial systems throughout the body, and not
just the lungs. Most importantly, he argued that the beat of the heart
produced a continuous circulation of blood through minute connections at
the extremities of the body. This is a conceptual leap that was quite
different from Ibn al-Nafis' refinement of the anatomy and bloodflow in
the heart and lungs."
This work, with its essentially correct exposition, slowly convinced
the medical world. However, Harvey was not able to identify the
capillary system connecting arteries and veins; these were later
discovered by Marcello Malpighi in 1661.
Illustration of a simple three-axis gimbal set; the center ring can be vertically fixed
A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal
pivot axes, may be used to allow an object mounted on the innermost
gimbal to remain independent of the rotation of its support (e.g.
vertical in the first animation). For example, on a ship, the gyroscopes, shipboard compasses, stoves, and even drink holders typically use gimbals to keep them upright with respect to the horizon despite the ship's pitching and rolling.
The gimbal suspension used for mounting compasses and the like is sometimes called a Cardan suspension after Italian mathematician and physicist Gerolamo Cardano
(1501–1576) who described it in detail. However, Cardano did not
invent the gimbal, nor did he claim to. The device has been known since
antiquity, first described in the 3rd c. BC by Philo of Byzantium, although some modern authors support the view that it may not have a single identifiable inventor.
The gimbal was first described by the Greek inventor Philo of Byzantium (280–220 BC).Philo described an eight-sided ink pot with an opening on each side,
which can be turned so that while any face is on top, a pen can be
dipped and inked — yet the ink never runs out through the holes of the
other sides. This was done by the suspension of the inkwell at the
center, which was mounted on a series of concentric metal rings so that
it remained stationary no matter which way the pot is turned.
In Ancient China, the Han dynasty (202 BC – 220 AD) inventor and mechanical engineer Ding Huan created a gimbal incense burner around 180 AD. There is a hint in the writing of the earlier Sima Xiangru (179–117 BC) that the gimbal existed in China since the 2nd century BC. There is mention during the Liang dynasty (502–557) that gimbals were used for hinges of doors and windows, while an artisan once presented a portable warming stove to Empress Wu Zetian (r. 690–705) which employed gimbals. Extant specimens of Chinese gimbals used for incense burners date to the early Tang dynasty (618–907), and were part of the silver-smithing tradition in China.
The authenticity of Philo's description of a cardan suspension
has been doubted by some authors on the ground that the part of Philo's Pneumatica which describes the use of the gimbal survived only in an Arabic translation of the early 9th century. Thus, as late as 1965, the sinologistJoseph Needham suspected Arab interpolation. However, Carra de Vaux, author of the French translation which still provides the basis for modern scholars, regards the Pneumatics as essentially genuine.
The historian of technology George Sarton (1959) also asserts that it
is safe to assume the Arabic version is a faithful copying of Philo's
original, and credits Philon explicitly with the invention. So does his colleague Michael Lewis (2001).
In fact, research by the latter scholar (1997) demonstrates that the
Arab copy contains sequences of Greek letters which fell out of use
after the 1st century, thereby strengthening the case that it is a
faithful copy of the Hellenistic original, a view recently also shared by the classicist Andrew Wilson (2002).
The ancient Roman author Athenaeus Mechanicus, writing during the reign of Augustus (30 BC–14 AD), described the military use of a gimbal-like mechanism, calling it "little ape" (pithêkion).
When preparing to attack coastal towns from the sea-side, military
engineers used to yoke merchant-ships together to take the siege
machines up to the walls. But to prevent the shipborne machinery from
rolling around the deck in heavy seas, Athenaeus advises that "you must
fix the pithêkion on the platform attached to the merchant-ships in the middle, so that the machine stays upright in any angle".
After antiquity, gimbals remained widely known in the Near East. In the Latin West, reference to the device appeared again in the 9th century recipe book called the Little Key of Painting' (mappae clavicula). The French inventor Villard de Honnecourt depicts a set of gimbals in his sketchbook (see right). In the early modern period, dry compasses were suspended in gimbals.
Applications
In a set of three gimbals mounted together, each offers a degree of freedom: roll, pitch and yaw
Inertial navigation
In inertial navigation, as applied to ships and submarines, a minimum of three gimbals are needed to allow an inertial navigation system
(stable table) to remain fixed in inertial space, compensating for
changes in the ship's yaw, pitch, and roll. In this application, the inertial measurement unit (IMU) is equipped with three orthogonally
mounted gyros to sense rotation about all axes in three-dimensional
space. The gyro outputs are kept to a null through drive motors on each
gimbal axis, to maintain the orientation of the IMU. To accomplish this,
the gyro error signals are passed through "resolvers"
mounted on the three gimbals, roll, pitch and yaw. These resolvers
perform an automatic matrix transformation according to each gimbal
angle, so that the required torques are delivered to the appropriate
gimbal axis. The yaw torques must be resolved by roll and pitch
transformations. The gimbal angle is never measured.
Similar sensing platforms are used on aircraft.
In inertial navigation systems, gimbal lock
may occur when vehicle rotation causes two of the three gimbal rings to
align with their pivot axes in a single plane. When this occurs, it is
no longer possible to maintain the sensing platform's orientation.
In spacecraft propulsion, rocket engines are generally mounted on a pair of gimbals to allow a single engine to vector thrust
about both the pitch and yaw axes; or sometimes just one axis is
provided per engine. To control roll, twin engines with differential pitch or yaw control signals are used to provide torque about the vehicle's roll axis.
Photography and imaging
Man using gimbal for smartphoneA Baker-Nunn satellite-tracking camera on an altitude-altitude-azimuth mount
Gimbals are also used to mount everything from small camera lenses to large photographic telescopes.
In portable photography equipment, single-axis gimbal heads are
used in order to allow a balanced movement for camera and lenses. This proves useful in wildlife photography as well as in any other case where very long and heavy telephoto lenses are adopted: a gimbal head rotates a lens around its center of gravity, thus allowing for easy and smooth manipulation while tracking moving subjects.
Gimbal systems are also used in scientific optics equipment. For
example, they are used to rotate a material sample along an axis to
study their angular dependence of optical properties.
Film and video
NEWTON S2 gimbal for remote control and 3-axis stabilization of a RED camera, Teradek lens motors and Angénieux lens
Handheld 3-axis gimbals are used in stabilization systems
designed to give the camera operator the independence of handheld
shooting without camera vibration or shake. There are two versions of
such stabilization systems: mechanical and motorized.
Mechanical gimbals have the sled, which includes the top stage where the camera is attached, the post
which in most models can be extended, with the monitor and batteries at
the bottom to counterbalance the camera weight. This is how the
Steadicam stays upright, by simply making the bottom slightly heavier
than the top, pivoting at the gimbal. This leaves the center of gravity
of the whole rig, however heavy it may be, exactly at the operator's
fingertip, allowing deft and finite control of the whole system with the
lightest of touches on the gimbal.
Powered by three brushless motors,
motorized gimbals have the ability to keep the camera level on all axes
as the camera operator moves the camera. An inertial measurement unit
(IMU) responds to movement and utilizes its three separate motors to
stabilize the camera. With the guidance of algorithms, the stabilizer is
able to notice the difference between deliberate movement such as pans
and tracking shots from unwanted shake. This allows the camera to seem
as if it is floating through the air, an effect achieved by a Steadicam in the past. Gimbals can be mounted to cars and other vehicles such as drones,
where vibrations or other unexpected movements would make tripods or
other camera mounts unacceptable. An example which is popular in the
live TV broadcast industry, is the Newton 3-axis camera gimbal.
Marine chronometers
The rate of a mechanical marine chronometer
is sensitive to its orientation. Because of this, chronometers were
normally mounted on gimbals, in order to isolate them from the rocking
motions of a ship at sea.
Gimbal with 3 axes of rotation. When two gimbals rotate around the same axis, the system loses one degree of freedom.
Gimbal lock is the loss of one degree of freedom in a
three-dimensional, three-gimbal mechanism that occurs when the axes of
two of the three gimbals are driven into a parallel configuration,
"locking" the system into rotation in a degenerate two-dimensional
space.
The word lock is misleading: no gimbal is restrained. All three
gimbals can still rotate freely about their respective axes of
suspension. Nevertheless, because of the parallel orientation of two of
the gimbals' axes there is no gimbal available to accommodate rotation
about one axis.
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