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in the body amounts to between 25 and 35 pounds ; and that about two ounces pass on from the heart at each pulsation. In this way, at 70 pulsations in a minute, 140 ounces will pass through the heart in a minute, or 8400 ounces in an hour. Hence the whole quantity of blood contained in the body, supposing it to be 25 pounds, will pass through the circulation in about three minutes, or about 20 times in an hour, or 480 times in a day. When we consider the same process in the larger species of animals, it strikes the mind still more forcibly. Dr. Hunter dissected a whale; and he relates that the aorta, which is the principal artery of the body, measured a foot in diameter. Ten or fifteen gallons of blood are thrown out of the heart at a stroke; what then must be the quantity of blood circulating through the whale in a day!

The structure of the heart, and the circulation of the blood, seem to be conducted on the same principles in man and in quadrupeds. We have just seen that in the whale it is similar ; and probably in fishes in general. The circulation of the blood, as it

appears

in the newt, a species of lizard, when seen through a good microscope, will illustrate what we have said on this subject.

The bodies of these animals, when very young, are so transparent, that the blood may be seen to flow briskly through every part, even into tlie toes, and to return from them. The newt has three small fins, near the head, which are divided like the leaves of a polypody or fern; and in every one of these branches, the blood may be traced, running to the end through the artery, and conveyed back again, by a vein of the same size with the artery, and laid in the same direction. In this part may be seen above thirty channels of blood running at once, like the divided streams of a great river, diffusing life and vigour.

Some insects have several hearts. If silk worms be examined, when full grown, there will be perceived a chain of hearts, running the whole length of their bodies; whilst many amphibious animals, frogs for example, have but one ventricle to the heart.

The chief distinction between the arteries and veins

lies in this, that the arteries convey the blood from the heart; the veins carry it back again. In order to effect this purpose, the veins are continued from the extremities of the arteries, and, in general, every artery is accompanied by its corresponding vein.

That we may clearly understand the subject before us, let us suppose two trees united to each other by the extremities of their branches at the top, and their trunks terminating at the same point at the bottom; each being hollow from the roots to the tips of the smallest twigs, and filled with a fluid which circulates incessantly from one through the other.

Let us imagine this, and we shall have a tolerably correct idea of the circulation of the blood through the vessels of the human body. Four large vessels, from which all the rest proceed, pass out from the base of the heart ; two of these are arteries, and the other two veins. The aorta is the principal artery, that distributes the main stream of the blood through innumerable ramifications, to all parts of the body; it arises from the left ventricle of the heart. The pulmonary artery originates from the "right ventricle, and enters the lungs, where its branches are spread out on the air-vessels: by this means the blood is acted upon by the air which we inhale, and undergoes a certain change which is essential to our well-being. All the veins, which bring the blood from the upper extremities, and from the head and heart, pass into a large vein called the descending vena cava ; those veins, which bring the blood from the lower extremities, pass into another large vein, called the ascending vena cava. These two large veins unite as they approach the heart, and open by one common orifice into the right auricle. The return of the blood is promoted by the action of the muscles, the pulsation of the arteries, and the valves which are formed in the veins. These valves are so nicely adapted to their design, that they admit the blood to flow from the extremities, but oppose its returning back towards them.

The circulation of the blood was first ascertained by Harvey, A.D. 1628.

RESPIRATION.

In forming the organs of respiration in the higher orders of animals, the Creator has had two great objects in view : the one, that of forming the voice; the other, that of completing the changes which are requisite for adapting the blood to the functions, which it is intended to perform in the animal economy.

The organs of respiration consists of the larynx, the trachea, or windpipe, and the lungs. The larynx is the projecting part, which you can see and feel at the upper part of the throat. It is the commencement of the windpipe, and is the organ in which the voice is formed. The windpipe is the tube which is connected with this, and is divided first into two, and then into smaller branches, called bronchia, which at last terminate in small cells, that form the mi ite structure of the lungs.

These organs can only be considered as subservient to the more immediate functions of respiration. There are other parts, which are necessary for carrying on the mechanical process of admitting and ejecting the air from the lungs, and these in man and quadrupeds are principally a very large and strong muscle, called the diaphragm, which separates the cavity of the abdomen from the thorax; and various small muscles which lie between the ribs.

The mechanism employed in dilatation and expansion is exceedingly simple. The contraction of the diaphragm forces down the abdominal viscera, and thus enlarges the cavity of the chest downwards, while the action of the muscles between the ribs raises them, and produces an expansion in another direction. The necessary effect of this increase of size is, that the air rushes into the windpipe, to supply the void which would otherwise occur ; and when the diaphragm and intercostal muscles cease to act, and become relaxed, the elasticity of the cartilaginous parts of the chest, but more particularly the tendency of the muscles of the abdomen to recover themselves, have the effect of diminishing the cavity of the chest, and of thus forcing

out from the lungs, the air which has been received by inspiration. The alternate dilatation and contraction of the chest, which thus take place, constitute the act of respiration, which is partly dependent on the will, and partly independent of it. The lungs are of a light, spongy texture, one in each cavity of the chest, capable of swimming in water, separable into subdivisions called lobes, and covered with a membrane called the pleura, which doubles back, and lines the cavity of the chest, as the peritoneum does the cavity of the abdomen. The lungs are very largely supplied with blood-vessels, of which some appear to be destined for the nourishment of the organ; but by far the principal part convey the blood from the right side of the heart, in order that it may, after minute division, and diffusion over the air-cells, be exposed to the influence of the external air, and be carried back to the heart in a proper state for nourishing the body.

The blood which passes from the right side of the heart into the lungs, is of a dark red colour. After circulating through the lungs, it becomes of a florid red, and has then been 'rendered fit for nutrition. In this progression through the lungs, it has been freely exposed to the air of the atmosphere, which is continually received and thrown out, by the alternate actions of inspiration and expiration.

Atmospheric air is composed of about twenty-one parts by measure of oxygen, or the respirable part ;. and seventy-nine parts of azote, called also nitrogen, or the unrespirable part, with a small portion, not exceeding two per cent, of carbonic acid gas. When an animal is confined in a certain quantity of atmospheric air, a part of the oxygen disappears, and an augmented quantity of carbonic acid gas is found to have been produced. Now, it is supposed by physiologists, that part of the oxygen is absorbed by the blood, giving it its florid red colour; and part unites with the carbon ot' the blood, forming carbonic acid gas. Though this is the generally received opinion, still some very accurate experiments make it probable that all the oxygen goes to the formation of carbonic acid gas.

Physiologists have differed very much as to the quantity of air taken in at each inspiration. It would appear, however, that about forty cubic inches of air are taken in at an ordinary inspiration; and if we suppose that we respire sixteen times in a minute, we shall respire, during the twenty-four hours, 921,600 cubic inches, or 533 cubic feet of air. This is an immense consumption of oxygen ; and it may seem extraordinary, that, considering the prodigious demands on the atmosphere, by the many millions of human beings who inhabit the earth, and the countless numbers of animals which require a constant supply of air, the oxygen should not be consumed, and the air itself contaminated. God, however, has wisely provided for the removal of what is noxious, from air, and for the supply of what is whole

Carbonic acid gas, which animals produce in respiration, and which likewise originates from fermentation and combustion, is capable of being absorbed by water. It is also, in certain circumstances, taken in by plants, of which it forms a part of the food, so that there is no danger of any deleterious superabundance. Plants, likewise, when exposed to the rays of the sun, exhale oxygen, which seems to arise from the decomposition of the absorbed carbonic acid gas, the carbon forming a part of the substance of the plant, and the oxygen, which had been united with it, being thrown

some.

out.

The influence, exercised by respiration, in the animal economy, is pretty much the same in all animals; but the mode, which we have described, principally applies to man and quadrupeds. In birds, there are some important modifications ; in fish the air is applied to the blood in the gills, through the medium of the water ; in amphibious animals, the principal characteristic is, that the whole of the blood does not circulate through the lungs, and that they can bear the interruption of respiration without injury; but in the insect tribe, and most of the inferior animals, there are various tubes, or tracheæ, which ramify over the body, and open externally by apertures, or stigmata, as they are called, by means of which the air is received and expelled : 80

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