The muscles of animal life, or striped muscles, include the whole class of voluntary muscles, the heart, and those muscles neither completely voluntary nor completely involuntary, etc. All these muscles are composed of fleshy bundles called fasciculi, inclosed in coverings of fibro-cellular tissue, by which each is at once connected with, and isolated from, those adjacent to it. Each bundle is again divided into smaller ones similarly ensheathed and similarly divisible; and so on, through an uncertain number of gradations, till we arrive at the primitive fasciculi or the muscular fibers peculiarly so called. Muscular fibers consist each of a tube or sheath of delicate structureless membrane, inclosing a number of filaments or fibrils. They are of cylindrical form, or of prismatic with one or more sides, according to the manner in which they are compressed by adjacent tissues. Their average diameter is about 1/500 of an inch, and their length never exceeds an inch and a half. The arrangement of the elementary substances in a muscular fiber (the sarcos element or protoplasm inclosed in the sarcolemna, the sheath) composing a muscular fiber may be compared to Volta's pile or an electric battery. In fact, both muscle and nerve are made up of electrical molecules, each of the two ends of which is negative--though the development of the electrical current is at present very imperfectly known. Besides, there is every reason to believe that the ground substance is similar in nature to ordinary protoplasm, but without the granular character commonly but not always exhibited. Blood-vessels are largely distributed in the substance of a muscle, carrying the materials necessary for its nourishment and chemico-vital changes, and there are also lymphatic vessels as in other vascular parts of the body. Nerves run through every muscle, by which the muscular contractions are called forth, and a low degree of muscular sensibility is conferred upon the muscular substance. The blood-vessels of the muscular tissues are extremely abundant, so that when they are successfully filled with a colored injection the fleshy parts of the muscle contrast strongly with its tendons. The arteries, accompanied by their veins, enter the muscle at various points and divide into branches, etc. The nerves of a voluntary muscle are of considerable size. Their branches pass between the fasciculi and repeatedly unite with each other in form of a plexus, which is for the most part confined to a small part of the length of the muscle, or muscular division, in which it lies. The voluntary muscles to which distinct names have been given in the system amount to about 240, and they naturally fall under the following four great divisions (the muscles are symmetrical and with few exceptions are in pairs): A. In the axial part of the body: 1. Muscles of the head and neck, 75 2. Muscles of the vertebral column and trunk, 51