The muscles have also the power of regenerating, although few experiments have been made except in those forms in which the whole leg can regenerate, yet there are a few observations that show that even in mammals, in which the leg or the arm cannot regenerate as a whole, a certain amount of regeneration of the muscles themselves may take place. It has been known for a long time that if a nerve is cut a new nerve grows out from the cut-end, and may extend to the organs supplied by that nerve. The process takes place more successfully if the peripheral part is left near the cut-end from which the new nerve grows. Whether this old part only serves to guide the new part to its proper destination, or whether it may also contribute something to the new nerve, as, for instance, cells for the new sheath, is not finally settled. The general opinion in regard to the origin of the new nerve fibres is that the central axis or fibril grows from the cut-end. That this power could have been acquired for each nerve as a result of its liability to injury is too improbable to discuss seriously. The central nervous system of the higher vertebrates seems to have very little power of regeneration, and although in some cases a wounded surface may be covered over and a small amount of connective tissue be formed, the development of new ganglion cells does not seem to occur. In other animals, as the earthworm, planarian, and even in the ascidian, as shown by Loeb, a new entire brain may develop after the removal of the old brain, or of that part of the body in which it is contained. This examination of the power of regeneration of internal organs in the vertebrates has shown that it is highly improbable that there can be any connection between their power of regeneration and their liability to injury. That the internal organs may be occasionally injured by bacteria, or by poisons made in the body, may be admitted, but that injuries from this source have been of sufficient frequency to establish a connection, if such were indeed possible, between their power of regeneration and their liability to injury from these causes is too improbable a view to give rise to much doubt. These results taken in connection with those discussed in the preceding chapter go far toward disproving the view that the power of regeneration has a connection with the liability of a part to injury. _HYPERTROPHY_