In an organism, circulation is the flowing of sap or blood through the veins or channels, by means of which the perpetual and simultaneous movements of composition and decomposition manifested in organic life are carried on. Although Galen, who had observed the opposite directions of the blood in the arteries and veins, may be said to have been upon the very point of discovering the circulation, the discovery was reserved for William Harvey, who in 1628 pointed out the continuity of the connections between the heart, arteries, and veins, the reverse directions taken by the blood in the different vessels, the arrangements of valves in the heart and veins so that the blood could flow only in one direction, and the necessity of the return of a large proportion of blood to the heart to maintain the supply.
In 1661 Malpighi exhibited microscopically the circulation in the web of a frog's foot, and showed that the blood passed from arteries to veins by capillaries or intermediate vessels. This finally established the theory with regard to animals, but the movements of sap in vegetables were only traced with difficulty and after numerous experiments.
Many physiologists were reluctant to ascribe the term 'circulation' to this portion of the economy of plants; but though sap, unlike the blood, does not exhibit movements in determinate vessels to and from a common centre, a definite course is observable. In the stem of a dicotyledonous tree, for example, the sap describes a sort of circle, passing upwards from the roots through the newer woody tissue to the leaves, where it is elaborated under the action of air and light; and thence descending through the bark towards the root, where what remains of it is either excreted or mixed with the new fluid, entering from the soil for a new period of circulation.
In infusorial animalcules the movement of the fluids of the body is maintained by that of the animal itself and by the disturbing influence of nutritive absorption. In the Coelentera (zoophytes, etc) the movement receives aid besides from the action of cilia on the inner walls of the body. The Annelids, as the earth-worm, possess contractile vessels traversing the length of the body. The Insects, Crustaceans, Myriapods, and Spiders have a dorsal tube, a portion of which may be specially developed as a heart. The blood is driven to the tissues, in some cases along arterial trunks, being distributed not in special vessels, but simply through the interstices of the tissues. From the tissues it is conveyed, it may be by special venous trunks to a venous sinus which surrounds the heart and opens into it by valvular apertures. The Mollusca have the heart provided with an auricle and a ventricle, as in the snail and whelk; two auricles, one on either side of the ventricle, as in the fresh-water mussel; or two auricles and two ventricles, as in the ark-shells. Among the ascidians, which stand low in that division of animals to which the molluscs belong, the remarkable phenomenon is encountered of an alternating current, which is rhythmically propelled for equal periods in opposite directions.
All vertebrated animals (except Amphioxus) have a heart, which in most fishes consists of an auricle and ventricle, but in the mud-fishes (Lepidosiren) there are two auricles and one ventricle; and this trilocular heart is found in the amphibians, and in most reptiles except the crocodiles, which, like birds and mammals, have a four-chambered organ consisting of two auricles and two ventricles. In these two last-named classes the venous and arterial blood are kept apart; in the trilocular hearts the two currents are mixed in the ventricle. Research Circulation
The Elasmobranchs (Elasmobranchii) is an order of fishes, including the sharks, dog-fishes, rays, and chimsera, in which the skull is not composed of distinct bones, but simply forms a kind of cartilaginous box, the vertebral column sometimes cartilaginous, sometimes consisting of distinct vertebrae, the integumentary skeleton in the form of placoid scales, the intestine being very short, and provided with a spiral valve. They have two pairs of fins (pectorals and ventrals), corresponding to the fore and hind limbs, and the ventral fins are close to the anus. The heart consists of an auricle, a ventricle, and a muscular arterial bulb. The gills are fixed, and form a number of pouches, which open internally into the pharynx, communicating outwardly by a series of apertures placed on the side of the neck. Research Elasmobranchs
Fish (Pisces) are an aquatic class of vertebrates, the lowest class of vertebrates. Fish may be briefly described as vertebrate animals living in water and respiring the air therein contained by means of gills or branchiae, having cold red blood, and a heart consisting of one auricle and one ventricle; and having those organs which take the form of limbs in the higher vertebrata represented by fins.
There are more kinds of fish than all other kinds of aquatic and terrestrial vertebrates put together. The smallest fish is the Trimmaton nanus, a goby of the Indian Ocean, which grows to about one centimetre long. The largest fish is the whaleshark, which may grow more than 12 metres long and weigh over 14 metric tons. It feeds on plankton and is completely harmless to most other fish and to human beings. The most dangerous fish weigh only a few kilograms. They include the deadly stonefish, whose poisonous spines can kill a human being in minutes.
Fish live almost anywhere there is water. They are found in the near- freezing waters of the Arctic and in the steaming waters of tropical jungles. They live in roaring mountain streams and in quiet underground rivers. Some fish make long journeys across the ocean. Others spend most of their life buried in sand on the bottom of the ocean. Most fish never leave water. Yet some fish are able to survive for months in dried-up riverbeds.
Fish have enormous importance to human beings. They provide food for millions of people. Fishing enthusiasts catch them for sport, and people keep them as pets. In addition, fish are important in the balance of nature. They eat plants and animals and, in turn, become food for plants and animals.
Fish thus help keep in balance the total number of plants and animals on the earth. All fish have two main features in common. (1) They have a backbone, and so they are vertebrates. (2) They breathe mainly by means of gills. Nearly all fish are also cold-blooded animals - that is, they cannot regulate their body temperature, which changes with the temperature of their surroundings. In addition, almost all fish have fins, which they use for swimming. All other water animals differ from fish in at least one of these ways. Dolphins, porpoises, and whales look like fish and have a backbone and fins, but they are mammals. Mammals breathe with lungs rather than gills. They are also warm-blooded - their body temperature remains about the same when the air or water temperature changes. Some water animals are called fish, but they do not have a backbone and so are not fish. These animals include jellyfish and starfish. Clams, crabs, lobsters, oysters, scallops, and shrimps are called shellfish. But they also lack a backbone.
The first fish appeared on the earth about 500 million years ago. They were the first animals to have a backbone. Most scientists believe that these early fish became the ancestors of all other vertebrates. Research Fish
The ear is the organ used for hearing. It converts sound into electrical impulses that are fed to the brain. The external ear is composed of the auricle (the pinna), and the auditory canal (the meatus auditorius externus). The Pinna or auricle surrounds the entrance to the auditory canal. It consists of cartilage covered by skin, with small muscles connecting it to the scalp. At the base of the ear is a fleshy lobe. The meatus auditorius is a canal about three centimeters long in the adult, partly bony and partly cartilaginous, leading from the pinna of the ear to the drum. The lining cells secrete the waxy substance found in the canal. In young children the canal is much shorter. The ear drum (tympanic membrane) is a thin oval-shaped membrane, inserted into a groove around the auditory canal. Normally it is white, glistening and somewhat transparent, so that some of the structures of the middle ear are partly visible when viewed through an auroscope. It separates the auditory canal from the middle ear.
The Tympanum or middle ear is a cavity within the temporal bone. It contains several important structures, including three small bones which connect the drum with the internal ear; they are the malleus or hammer, the incus or anvil, and the stapes or stirrupbone. They transmit the vibrations of sound waves to the inner ear. The Eustachian Tube is a channel of communication between the tympanum and the upper part of the pharynx. It admits air from the throat to the tympanum and so maintains an equal pressure on both sides of the drum. The Labyrinth or internal ear is a series of chambers through the petrous bone, comprising the vestibule, a three-cornered cavity within the tympanum; the semicircular canals communicating with the vestibule; and the cochlea, which makes two and a half turns around an axis called the modiolus. The human ear is capable of detecting sounds in the frequency range 20 hz to 20 khz, approximately. Research Ear
The eustachian valve is one of the valves of the heart, lying at the point where the inferior vena cava empties itself into the right auricle. Research Eustachian Valve
The heart is a hollow pear-shaped muscular organ placed between the lungs in the middle of the chest that pumps blood through the body, supplying cells with oxygen and nutrients.
The heart in humans, quadrupeds, birds, and some reptiles is composed of four cavities, two auricles and two ventricles. It is enveloped in a membrane called the pericardium, and is situated toward the left of the cavity of the chest, between the lungs. With each beat the apex of the heart strikes against the wall of the chest in the space between the 5th and 6th ribs, a little below and to the right of the left nipple. The right auricle communicates with the right ventricle, besides which there are in it three openings, that of the vena cava inferior, that of the vena cava superior, and that of the coronary vein. The communication between this auricle and ventricle is closed by a valve when the ventricle contracts. The right ventricle communicates with the pulmonary artery, the opening into the artery being guarded by a valve formed of three flaps. When these are brought together they interrupt the communication between the ventricle and the artery. The left auricle communicates through a valved opening with the left ventricle, and contains the orifices of the four pulmonary veins. The left ventricle, besides the communication with the left auricle, contains the orifice of the aorta, also provided with a valve similar to that of the pulmonary artery.
The auricles and ventricle of one side are separated from those of the other by a complete muscular partition, the septum cordis. The valves at the openings of the arteries are called semilunar, that at the orifice of the right auricle tricuspid, that at the orifice of the left auricle mitral, and that at the orifice of the vena cava inferior the Eustachian valve.
The heart is formed of a firm thick muscular tissue, composed of fibres interlacing so as to form a figure of eight. It also contains nerves and vessels.
The arteries carry the blood from the heart to all parts of the body. They terminate in the capillary vessels, a series of extremely minute tubes which pass over into the veins.
The veins are the channels by which the blood passes back from the body to the right auricle of the heart. The blood which is returned from the veins is purplish red, from the presence of carbon dioxide and deficiency in oxygen, and is called venous; that which leaves the heart is bright red, being oxygenated, and is called arterial.
The venous blood parts with its carbon dioxide and receives new supplies of oxygen in the capillary system of the lungs, flows into the pulmonary veins, thence into the left cavities of the heart, thence it passes into the aorta, and is transmitted to all parts of the body, returning to the veins by the capillary system. It has now become venous, passes through the veins from the extremities towards the heart, receiving the chyle and the lymph, and is emptied into the right cavities of that organ, which returns it through the pulmonary artery to the capillary vessels of the lungs, where it is subjected to the influence of the air, resumes the qualities of red or arterial blood, and is ready for a new course.
The mechanism of the circulation is as follows: The blood contained in the two venae cavae is poured into the right auricle, which contracts, and thus forces the fluid to escape; but the venae cavae oppose to its backward passage the column of blood which they contain, and it must therefore pass into the right ventricle. The ventricle then contracts, and the tricuspid valve closing the passage through which the liquid entered, it is forced into the pulmonary artery, along which it must flow (return to the ventricle being prevented by the semi-lunar valve) into the capillary system of the lungs, whence it passes into the pulmonary veins, which pour it into the left auricle by four orifices.
The contraction of the auricle impels it into the left ventricle, by which it is driven forward into the aorta (the mitral valve preventing its return into the auricle), and thence into the general circulation. The two auricles contract and relax simultaneously with each other, as do also the two ventricles. The relaxation is called diastole; the contraction systole.
The quantity of blood projected at each systole is generally estimated at six ounces. The causes of the alternate contraction and relaxation are entirely involuntary and dependent on the nervous system to a large extent. The systole of the ventricles is the cause of the motion of the blood in the arteries, which dilate with each wave driven into them.
The heart is the seat of various and generally dangerous diseases. One of these is pericarditis or inflammation of the pericardium, the double lining membrane or bag enveloping the heart. The cause of this disease may be exposure to cold, or an injury, or it may be complicated with other diseases. Inflammation of the inner lining is termed endocarditis.
Valvular disease is a common affection of the heart, the valves becoming thickened, contracted, rigid, or otherwise affected, so that they cannot properly perform their duty. The mitral valve, for instance, may become too narrow and contracted, and the result is that all the blood does not pass into the aorta. In other cases of valvular disease, the same result follows, that is imperfect depletion of the ventricles and auricles, the return of blood being termed regurgitation. The heart consequently becomes weakened, while the entire system suffers.
Overgrowth or hypertrophy and dilatation are frequent results of valvular disease. The use of digitalis is often successful in strengthening and soothing the heart. Certain diseases produce atrophy, in which the heart becomes feeble in action, while fatty degeneration occurs, when the muscular fibres are replaced by oleaginous particles. Research Heart
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Auricle
