Although the heart in all animals is formed on the same general principle, and for the same purpose, yet the economy of some animals admits of a greater simplicity in the conformation of this organ, than others. The most simple kind of heart is composed of one cavity, with a tube entering into it, by which it receives the blood, and another passing out of it, by which the blood is conveyed over the body. The next simple heart is composed of two cavities, an auricle, which receives the blood, and propels it into a ventricle, which diffuses it over the body. Another kind of heart is composed of three cavities; two auricles, and one ventricle; one auricle receiving the blood from the lungs, the other from the body generally; the blood from these two sources is mixed together in a single ventricle. This structure we find in some amphibious animals, in which it is not necessary that the blood should circulate with so much influence from the oxygenous part of the atmosphere, as in other animals. Accordingly we find the heart adapted to transmit only one half of the blood through the lungs at each circulation, whilst in more perfect animals the whole mass passes by this route. The last kind of heart is formed of four cavities, two auricles and two ventricles, and is the most perfect apparatus as it is found in man, and quadrupeds generally.—It must, however, be considered as composed of two distinct parts, or two simple hearts adhering together, and performing distinct parts of the circulation; and one part intended to receive the blood from the body, and circulate it through the lungs; the other part to receive the blood from the lungs, and propel it over the rest of the body. It is better suited to the economy of some animals, as the cuttle fish, that these parts should be separated to a considerable distance from each other. The reason why the heart is formed of two parts in most animals is, that it is necessary that the blood should receive the impulse of the heart twice, first to propel it through the lungs, next to propel it over the rest of the body.
The blood is conveyed from the heart to every part of the body, by means of elastic tubes, called arteries. These arise from the ventricles of the heart by two large trunks, which branch out in every part of the body, into arteries of great minuteness, conveying the blood from the heart to its most distant parts, so that it is impossible to wound any part of the body with the finest point, without opening one of these vessels. This gives a good idea of their minuteness.
From the minute termination of the arteries, begins a second set of vessels, the veins, which, having a contrary course, return the blood from every part of the body into the auricles of the heart.
The larger arteries and veins, near the heart, differ very much from each other in their structure and action. This difference, however, does not descend to their minute ramifications, which must be considered as having the same structure, and performing the same office, the one passing into the other by such imperceptible degrees, that we cannot mark where the one terminates or the other begins.
If we consider these tubes as subservient to the circulation of the blood, we shall see the necessity of certain principles entering into their structure. As the blood is forcibly thrown from the heart, these vessels must be distended; one of their properties therefore, must be a capability of being distended, which is given to them by elastic matter entering into their composition. As the vessels, however, are not to remain in a distended state, a power of reaction is added, which arises also out of their elasticity, and assists in propelling the blood forwards.
Thus the elastic matter allows the vessels to be distended to a certain degree, and also reduces them to a smaller size. But it is necessary that the heart shall be assisted considerably, in the circulation of the blood, by a contractile power of the vessels themselves; and the same quantity of blood is not to circulate in the same body at all times, for animals are liable to frequent injuries, by which the quantities of blood in their bodies may be very suddenly reduced. Hence the vessels have given to them a further power of contraction to assist the heart, and accommodate themselves, under certain circumstances, to a smaller quantity of blood. For this purpose, a muscular structure is added to them, which is present in largest proportion in the smaller arteries; by this means, they are enabled so far to withstand the power of the heart, as to shut their cavities, and prevent the escape of blood when divided, forming one of the means by which the effusion of blood is spontaneously checked in living animals. And it may be remarked, that this power, for the purpose of self-preservation, is extended to larger arteries in the brute creation; for Mr. Hunter found, that the flow of blood from the large artery in the neck of an ass was checked by an exertion of this power, whilst every one knows that its division in man is fatal.
Besides these parts, arteries have an internal lining, which is perfectly smooth, and of considerable density, that the blood may circulate with as little resistance, and be contained as completely as possible within its proper channels.
The same observations will apply to the veins, though some of their properties are less strongly marked. They possess an elastic power capable of distension and reaction, a muscular structure endowed with contractility, and an internal lining over which the blood circulates with as little resistance as possible. By these powers the blood is circulated through every part of the body with great velocity. According to the best calculations, the heart alone exerts a power equal to the pressure of 51½ pounds, which propels the blood through the arteries at the velocity of 149 feet in a minute; in which time it expels from its cavities about 160 ounces.
Thus all animals are provided with an organ for propelling the blood, by certain channels, to the different parts of the body; but, as the functions of these parts are various, they require to be visited by very different proportions of blood, according to their activity or powers of life. Some parts of the body may be said to be inert, and merely possessed of a principle of life, to connect them with the other organs of the body, as parts of a living system, and to enable them to go through certain processes in their healthy and diseased states. Other parts are formed for active functions, and possess great sensibility. It is accordingly observed, that a smaller quantity of blood is distributed to bones, tendons, and similar inert parts, than to muscles and glands, whose exertions are more considerable.
This then is the general apparatus in perfect animals, by which the blood performs its circulation through the various parts of the body, but during its course it is subject to constant exhaustion from various sources. It is converted in its passage into the nature of all the component parts of the body, and has the different secreted fluids derived from it, and these processes go on with more activity in a young, than an adult person: hence we see the necessity of a constant supply of materials to the blood, and this in the greatest proportion at an early period of life.