Exertion, under certain regulations, produces a particular state of the muscles, the parts of motion, and of the nerves, the blood, and the blood-vessels, by which the muscles are supplied. Neither anatomy nor physiology is able to describe the change which those parts undergo in training. The eye, indeed, discovers a difference in the texture and the color of the muscles. Those which have been much in use are redder, harder, and tougher, than those that have had little to do. They contain more blood, and that blood is of a more decided red color. They are also a little larger, when compared with a corresponding muscle of less work. More than this dissection does not reveal. It is known, without any dissection, that the instruments of motion exist in different states; that in one state their action is slow and feeble; in another state it is rapid and powerful • and that in certain states they can maintain their action for a much longer time than in certain other states.

For practical purposes it is not perhaps of much consequence to learn all the changes which the muscles, the blood, the blood-vessels, and the nerves, must undergo, before the horse can possess the condition which his work demands. It may be enough to know that the condition, in whatever it may consist, can be conferred only by exertion. There are numerous auxiliaries, and various modes of giving and of regulating exertion; but until it has produced the requisite alteration in the muscles, and their appendages, there can never be any remarkable degree of speed nor endurance.