Page:Popular Science Monthly Volume 43.djvu/390

376 to our knowledge of the nervous system is perhaps the most important which has been made during the last generation. It teaches us that the nervous system comprises two sets of nerve cells and fibers, which differ not only in their situation, but also in their development and distribution. We are already in a position to say that the entire physiology of the brain must henceforth be based upon this discovery of the independence of the ganglionic system, because the same laws can not apply without change to structures so differently organized as are the two portions which we have briefly characterized, and there can be no doubt that the functions are as fundamentally divergent as is the organization. It is, however, still too soon for cerebral physiology to have remodeled itself, but that remodeling must follow, since physiology always bases itself on the anatomical facts.

Besides the two classes of nerve fibers, the medullary and ganglionic, we may have to add a third. In the organs of special sense (sight, hearing, smell, and taste) there are found the peculiar sensory cells, which all present two special features: First, they have characteristic modifications of cellular structure, by which they are adapted to receive sensory impressions; second, they are each united with a single nerve fiber. It has long been, and indeed still is, the prevalent theory that the nerve fiber arose from the brain, grew to the cell, and united with it. Merkel was, I think, the first to suggest that the sensory cells are also true nerve cells, the nerve fiber springing from them and growing to the brain. This view has been brought into fresh prominence by the discovery made by Michael von Lenhossék that Merkel's supposition is true in the case of the earthworm, which has cells scattered in its skin, each cell giving rise to a nerve fiber, which must arise from the sensory cell since it is connected with no other cell, although it enters the central nervous system and there ramifies.

—For the recognition of the three sets of nerve roots also we are indebted to the researches of His, published in 1888. Previous to that time anatomists recognized two roots only—the posterior or dorsal roots, and the anterior or ventral roots. In the spinal cord it was easy to maintain Bell's law, that the posterior roots are sensory; the anterior, motor or efferent. The cephalic nerves, however, could not be brought into accord with this law, because of numerous difficulties, of which one may be mentioned as an example. The nerve called the facial was found physiologically to be both sensory and motor, and yet was shown embryologically to correspond to a posterior root. Through His we learned that the cephalic nerves corresponding to the posterior roots have in reality compound roots, being double. In fact, the nerves of the class referred to consist each of a bundle of ganglionic fibers which enter the brain and branch in its