376 THE POPULAR SCIENCE MONTHLY. 



to our knowledge of the nervous system is perhaps the most im- 

 portant 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 posi- 

 tion to say that the entire physiology of the brain must hence- 

 forth 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 por- 

 tions 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 gan- 

 glionic, we may have to add a third. In the organs of special sense 

 (sight, hearing, smell, and taste) there are found the peculiar sen- 

 sory 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 discov- 

 ery made by Michael von Lenhosse'k that MerkeFs 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. 



The Second Discovery. — 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 dor- 





