Muscle. The muscles are the contractile tissues of the animal body, and the power of altering their shape which they manifest is the means by which the various movements incidental to respiration, the circulation of the blood, etc., and the alterations of the position of the various parts of the skeleton with respect to other parts are brought about. There are three kinds of muscle. Unstriped muscle is also called involuntary muscle, as it is concerned in those movements which are not regulated by the will, e.g. contraction of the bladder, alterations in the calibre of blood-vessels, etc. Unstriped muscle is made up of spindle-shaped nucleated cells, which are grouped in bundles, and these again may be aggregated together in the form of a membrane, such as occurs in the muscular coat of the bladder. Hea/rt muscle is intermediate in character between the last-named variety and that which will be subsequently described; though it contracts apart from the influence of the will, it resembles to some extent the ordinary voluntary muscle; it possesses some degree of striation when seen under the microscope, but the fibres are branched; they possess no distinct limiting membrane, and the nucleus is situated in the body of the fibre. Striated, or ordinary voluntary, muscle is made up of fibres which possess a limiting membrane, the sarcolemma, and immediately beneath this membrane, lying external to the main body of the fibre, there is found here and there a nucleus. When examined microscopically the fibres are seen to possess a distinct transverse striation, due to the muscle fibre possessing alternately arranged dim and bright bands; this appearance is more markedly developed in the muscles of certain invertebrates, particularly insects, than it is in man. Chemically, muscle consists of certain proteid substances, extractive bodies (kreatin, etc.), salts and water. The chief proteid of muscle is a body known as myosinogen. When a muscle is removed from the body it dies and undergoes what is known as rigor mortis - that is to say, it becomes stiff and rigid - this change being accompanied by the development of an acid reaction, and by the conversion of the myosinogen of the living muscle into myosin. This alteration of the muscle substance is closely analogous to the phenomenon of coagulation in blood, and myosin is nearly allied in composition to fibrin. The changes which occur when a muscle contracts have been attentively studied of recent years. It is found that heat is developed, sound is produced, certain changes in the microscopical appearance of the fibre occur, there is an evolution of carbonic acid gas, a taking-in of oxygen and the development of a body known as sarcolactic acid, and lastly there is an alteration in the electrical condition of the muscle, what is known as a current of action being developed.