evidence that the differences between muscular fibers, such as those of the heart, which contract normally with what we call a spontaneous beat, and the fibers of the skeletal muscles, which only, under ordinary circumstances, contract when excited through their motor nerves, are not so deep-seated as was at one time supposed, since the addition of simple inorganic bodies to the living muscular substance, or their subtraction from it, can alter its behavior in this regard. The experiments of Langendorff, Porter and others on the action of the isolated hearts of warm-blooded animals, which, after being cut out of the body, can be kept alive for several hours by feeding them through their arteries with warm blood from a reservoir, have strengthened the belief that the essential cause of the heart-beat is to be sought in the muscular fibers and not in the nerve-cells present in certain portions of the organ.
With respect to the nerve-cell, research is at present largely concentrated upon the study of its minute structure. Among the numerous methods of staining employed for this purpose two deserve especial mention: the method of Golgi, which is peculiarly useful for bringing out the processes or branches of nerve-cells, and the method of Nissl, which is of great service in the investigation of the body of the cell. A typical nerve-cell when impregnated with a salt of silver, according to Golgi's method, exhibits a wonderful profusion of bifurcating processes, picked out in black like the sharply shadowed branches of a leafless tree under an electric light. But, however intricately the branches of neighboring cells may mingle and intertwine, they do not in general run into, or fuse with, each other, any more than the interlocking boughs of neighboring trees in a forest. By demonstrating this important fact the method of Golgi has revolutionized our ideas of the architecture of the nervous system.
The significance of the peculiar angular or spindle-shaped bodies in the protoplasm of the nerve-cell, which have been revealed by Nissl's method of staining with methylene-blue, is at present arousing the greatest interest. That they have some important relation to the nutrition of the cell seems evident. For when the latter is severed from that one of its processes (the axone) which constitutes the essential part of the nerve-fiber that springs from the cell, the Nissl bodies break up, and either disappear or are dispersed in the form of very minute granules of stainable material in the protoplasm. At the same time the cell becomes swollen, its nucleus is displaced to one side, and it may even atrophy entirely and disappear. As a rule, however, after several months it recovers its normal structure. The administration of considerable quantities of alcohol and other drugs causes a similar effect on the Nissl bodies. It has been known for half a century that the axone degenerates when cut off from the cell of which it is a process.
