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 Dipnoi are an order of fishes, including only the singular mud-fishes (Lepidosiren), important as exhibiting the transition, between fishes and the amphibia. Formerly Lepidosiren was reckoned the lowest of the amphibia, now it constitutes the highest order of fishes. The body is fish-like in shape, covered with small horny scales of a cycloid character; the pectoral and ventral fins are represented by two pairs of long filiform organs; the heart has two auricles and one ventricle, and the respiratory organs are twofold, consisting of ordinary gills opening externally, and of true lungs - formed by the modified swimming-bladder - communicating with the oesophagus by means of an air-duct or trachea, whence the name. They are also called Protopteri. The combination of respiratory organs is similar to that which is presented by the tailed amphibians with persistent gills (perennibranchiate), as the axolotl. This interesting group is allied to the ganoids through the Ceratodus of Queensland. The Lepidosiren paradoxa is found in the Amazon; Lepidosiren annectens in the Gambia. Research Dipnoi
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 aortic semilunar valve is situated at the beginning of the aorta. This valve has three delicate cusps, or pockets, which permitblood flow only in one direction, allowing blood to flow out of the left ventricle up into the aorta, but prevents backflow into the ventricle. When the heart refills, the crescent-shaped valve balloons out, sealed along the edge to prevent the blood from flowing backward. The blood's only exit from the ventricle is through the semilunar valves, so named for their crescent shaped cusps. Research Aortic Semilunar Valve
Arteries are muscular and elastic-walled vessels that form a network to carry oxygen-rich blood from the heart to all parts of the body. Smaller branches called arterioles extend from the arteries and connect to even smaller branches called metarterioles which deliver the blood to the capillaries. The exchange of oxygen and carbon dioxide between blood and body cells takes place through the thin walls of the capillaries.
There are two principal arteries or arterial trunks: the aorta, which rises from the left ventricle of the heart and ramifies through the whole body, sending off great branches to the head, neck, and upper limbs, and downwards to the lower limbs, etc; and the pulmonary artery, which conveys venous blood from the right ventricle to the lungs, to be purified in the process of respiration. Research Artery
The left atrium is a small upper cavity of the heart. Oxygen rich blood returns from the lungs through the four pulmonary veins into the smooth chamber of the left atrium. The chamber is constructed of two overlapping layers of muscle: a superficial layer and an inner layer, composed of many small bundles. The wall of the chamber is slightly thicker and more powerful than the right atrium. As the heart contracts (ventricular systole), blood flows into the ascending aorta through the aortic arch. As the heart relaxes (ventriclediastole), the blood flows through the mitral valve to the left ventricle. The right atrium is a small upper cavity of the heart that holds about three-and-a-half tablespoons of blood. It serves as the receiving chamber for all the venous blood (short of oxygen and laden with carbon dioxide) returning through the superior and inferior vena cava, and from many minute blood vessels that drainblood from the walls of the chamber itself. The right atrium is slightly larger than the left atrium, which is slightly more powerful. The walls of the right atrium are less than an eighth of an inch thick. Two layers of muscle form the wall. The superficial layer spans both atria, and the inner layer, composed of many small bundles, arches over the atrial cavity at right angles to the superficial layer. As the heart contracts (ventricular systole), the blood is pushed through the pulmonary valve into pulmonary circulation. As the heart relaxes (ventricular diastole), the blood exits the right atrium through the tricuspid valve to the right ventricle. In the upper part of the right atrium there is a small patch of special hearttissue called the sinus node or the sinoatrial node. It is the heartspacemaker, triggering the heartbeat and establishes its rate. Research Atrium
The caudate nucleus and the lentiform nucleus are both part of the striated body (corpus striatum) of white and grey nerve fibres located just laterally from the fornix within the brain. The lentiform nucleus, or nucleus lenticularis, is the extra-ventricular portion, as it is imbedded in the white material of the cerebral hemisphere. The caudate nucleus, or nucleus caudatus, extends from the outer side of the optic thalamus to the roof of the decending cornu of the lateral ventricle, at the apex of the temporal lobe. Research Caudate Nucleus
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Circulation
