This fluid is distributed over every portion of the frame. Without its agency there is nothing done in any part of the body; and, in performing its varied duties, it suffers some alteration, which renders it unfit to reproduce the same effects, or perform the same functions, until it has acquired something it has lost, and parted with something it has gained. The purification, or regeneration, takes place chiefly in the lungs. To these organs, which almost entirely fill the chest, the blood must be conducted. It is collected from every tissue, by veins infinitely numerous and small; too numerous to be counted, and too small to be traced even with the aid of optical instruments. These, as they approach the heart, concentrate, become larger and fewer, till they end in two main trunks of very large size, which pour their contents into a cavity on the right side of the heart. The heart sends this blood to the lungs, by one large tube. This, running into the substance of the lungs, divides and subdivides, till its branches become so numerous and minute that they can not be distinguished from the tissue in which they are embedded. Nevertheless these veins form but a small portion of the lungs.

There is another set of vessels, equally minute and numerous, for taking the blood back to the heart. In its passage through the lungs, the blood is exposed to the air, which acts upon it, though covered from actual contact. The blood is thus changed in composition. It is purified, losing something or gaining something, and is ready again to perform the duties of which it had previously become incapable. In this state it is collected from the lungs, and taken to the cavity in the left side of the heart, whence it is sent by another set of tubes to be distributed over the body. These are termed arteries; as they pass into the substance of parts, their ultimate arrangement can not be traced. In the hidden recesses, the blood performs its functions. There it produces changes on the tissue, and is itself changed. It suffers some deterioration, or alteration, which can not be rectified till it reaches the lungs, to which the veins collect and carry it.

The blood is in constant motion. It is not all altered at one time, nor at one place At some particular places the alteration may be greater than at others; but the best and the worst are mingled together on their road to the heart. Under ordinary circumstances, the purification keeps pace with the deterioration. Both go on simultaneously, and to an equal degree. But in some cases the equilibrium is deranged.

Muscular Exertion produces at least four important changes. It quickens the circulation; it quickens the breathing; it increases the formation of heat; and it produces per spiration. The muscles are the active instruments of motion. They act by alternate contraction and relaxation; their active state is that of contraction. They shorten, and their ends being fixed to different bones, motion takes place from the joints. The animal wills to move, and the muscles instantly produce the motion desired. The direction and velocity, the force and duration of the motion, are regulated entirely by the will of the animal. But, in order that the muscles may obey, it is an indispensable condition that they have an abundant supply of pure blood. In action they consume more than at rest.

Quickness of the Circulation is therefore a necessary consequence of muscular exertion. The muscles demand more blood; and the heart hastens to furnish it by performing double, treble, or more than treble its usual number of strokes When the horse if at rest, the heart contracts from thirty to forty times in a minute. Every contraction drives a colunm of blood through the arteries. At slow work the heart may beat from fifty to seventy times per minute; but at fast work it sometimes makes more than one hundred and forty strokes in a minute.

Quickness of the Breathing occurs almost simultaneously with the quickness of the circulation. There is a little time, however, it may be only a few seconds, between them; the circulation has the start. Acceleration of the breathing follows, in order that the blood may be purified as fast as it is circulated. At rest, the horse respires from six to eight times per minute; at slow work he may breathe twice as fast, and at very fast work, he may respire more than one hundred and thirty times per minute. The velocity of the blood must keep pace with the exertion of the muscles, and the respiration must quicken as the circulation quickens. The action of each is, in a certain measure, influenced by that of the. other, but each is also limited in its individual powers. The muscles can not act if the heart do not give them sufficient blood; the heart can not give the blood if the lungs do not purify it; but the muscles may tire, even though well supplied by blood; or the heart may tire, though the lungs continue vigorous.

Deficiency in either deranges the others.

Exertion may raise the pulse to one hundred and forty, and the breathing to one hundred and thirty; but at this rate, neither the heart nor the lungs can work long. After a period, which varies with the condition of the horse, the blood begins to accumulate in the right side of the heart. It is difficult to say what part is first in fault. The heart may be exhausted, unable to force the blood through the lungs; or the lungs may be unable to purify and transmit the blood as fast as the heart sends it; or the muscles which produce breathing may tire, and become unable to expand the chest, sufficiently to admit the blood and the air into the lungs; or, possibly, heart, lungs, and muscles, may all be at fault, some more, some less : whichever way it happen, the blood begins to accumulate, first in the right side of the heart, and then in the lungs. After this stagnation commences, the horse is not able to go much further. The muscles do not receive enough of blood; and that which they do receive is not good. The obstruction in the lungs forbids perfect purification. The horse becomes feeble, is disposed to slacken his pace, and some stand still before they are very much distressed.

But such is the disposition of certain horses; one will run on till he is blind, staggering, and stumbling; at last he falls, and rises no more. He dies suffocated. Upon dissection, the lungs are found so gorged with blood that almost no air could enter them.

At the first indications of distress the horse should be pulled up, or his pace should be slackened; half a minute may be sufficient to restore strength to the heart, the lungs, or the muscles, whichever be in fault; the stagnation or accumulation ceases, and the blood passes on free and pure.

An increased formation of Heat is the third effect of muscular exertion. The surface of the body becomes warm or hot; more than the usual quantity of heat is evolved. It has never been supposed that this is a necessary or useful consequence of exertion. Acceleration of the blood and of the breathing must take place in order that the muscles may produce progression. But it is not believed that an extra quantity of heat is useful either as an assistant or as a principal. It is well known that fast work does least mischief in cool or cold weather; and it appears that there is a contrivance almost for the express purpose of removing the superfluous heat. Most probably the evolution of heat is an unavoidable result of increased velocity in the circulation.

Perspiration is the fourth effect of exertion. By this process the "body is relieved from superfluous heat, and superfluous fluid. It is always refreshing. It enables the horse to perform his work with less distress; but when he has little superfluous fluid in him it always produces subsequent exhaustion. A fat or plump horse may be all the better of a good sweat; he may be fitter for his work next day than if he had not perspired. A very poor horse can not so well afford such a loss of fluid; the more he sweats to-day, the less spirit and strength ho has to-morrow. Both, however, are refreshed, though not perhaps in equal degrees, by perspiring at their work. In both, the perspiration combines with the superfluous heat, and carries it off in vapor. The evaporation regulates the heat of the surface. If it were possible to confine the heat which rapid exertion produces, it is probable the horse would soon be fevered. But it is not possible to do this, for whenever the skin becomes very warm, perspiration follows almost immediately.

Some horsemen, and especially, I believe, post-boys and stage-coachmen, are in the habit of throwing a pailful of cold water over the horse's body in the middle of a long stage on a hot day. Most people would regard this as a very violent and thoughtless proceeding. To deluge a horse with cold water, when reeking hot, and perspiring at every pore, ap pears to be a dangerous practice I can not speak from very extensive experience of this, but so far as I have been able to see, there is no danger in the case, so long as two rules are observed : the effusion must not be carried so far as to make the skin perceptibly cold; - and the horse must be put in motion directly after it is done. One or at most two bucket-fuls may be dashed over the body, as equally as possible; and the horse should immediately resume his journey; or, if his journey be over, the water must be scraped off, and the horse moved about till he be quite dry. The danger lies in letting him stand till he shivers. With these precautions, I have never seen the cold effusion do any harm, and I know well that it is highly refreshing to a heated and travel-worn horse, on a hot day.

The water withdraws the redundant heat, which oppresses the horse, and which he can get quit of only by a process comparatively slow in its operation and expensive to the system. The fat, plump horse, having plenty of superfluous fluid to spare, may not be so much in need of the cold effusion, but he also is much refreshed by it, particularly after he has already perspired copiously. After the temperature of the skin is fast sinking to its natural standard effusion is both useless and dangerous.

Acceleration of the circulation and of respiration, the generation of heat, and perspiration, are the immediate and most important effects of exertion. But there are other changes, which can not be distinctly traced, either in number or in order. The few that can be described do not appear to demand any notice but what they obtain in other parts of this work. A minute analysis is not necessary, though it might be interesting, and to the practitioner useful. It may be sufficient to observe in this place, that the nerves, the bloodvessels, the muscles, tendons, ligaments, and joints, undergo a slight change of state every time the horse is put to work. With some of these parts the alteration becomes apparent only after the change has been produced often, and at short intervals. The change of state, in whatever it may consist, is beneficial to a horse that has been long idle. By degrees is renders all the parts better able to perform their duties. Under proper management the alteration goes on progressively, until each part and each organ have attained all the improvement of which they are susceptible.

When muscular exertion is pushed beyond a certain point, an injurious alteration takes place in soma of the organs connected with motion.

The improvement of motive parts is considered in the next section; the deterioration in that which follows it.