382 THE POPULAR SCIENCE MONTHLY. 



brain or spinal cord. Most all the nerve fibers produced within 

 the brain enter the ventral territory, for in this territory we observe 

 not only the fibers which it obviously must include — namely, 

 those which are produced by the nerve cells of the ventral zone — 

 but also the nerve fibers produced by the nerve cells of the dorsal 

 zone. So far as at present known, the nerve cells of the dorsal 

 zone all produce nerve fibers, but these fibers always pass into the 

 ventral division of the nervous system. These fibers of dorsal 

 origin are the chief, perhaps the only ones, which are commis- 

 sural — that is to say, which pass to the opposite side of the brain ; 

 others of these fibers take longitudinal courses within the ven- 

 tral zone ; while still others participate in the formation of the 

 nearest ventral (or anterior) nerve roots. If, therefore, we as- 

 sume that the sensory nerve impulses are carried into the dorsal 

 zone and there transferred to the medullary nerve cells, we must 

 conclude that from those cells the impulse may be sent along me- 

 dullary fibers either into the opposite side, or up and down the 

 ventral zone, or into a neighboring nerve root. The center of 

 divergence is the dorsal zone, but the actual divergence of the 

 fibers takes place in the ventral zone. 



Although the ventral zone receives medullary fibers and itself 

 produces nerve fibers, it sends, so far as yet observed, no fiber into 

 the dorsal zone, but all the fibers which leave the ventral zone 

 form nerve roots and leave the nervous system altogether. These 

 roots, as we have already learned, are in two sets — the lateral and 

 ventral. 



Summary. — The numerous facts which we have marshaled in 

 hasty review so greatly widen our knowledge of the nervous sys- 

 tem that it is important to render them as clear as possible. If 

 what has been presented be critically considered, it will be found 

 that what we have gained is an enormous accession of knowledge 

 in regard to the nature, origin, distribution, and connections of 

 nerve fibers. In order to make the typical variations of nerve 

 fibers as evident as possible, I have constructed the accompanying 

 diagram, which is, I think, correct for all which it attempts to 

 give. We notice : First, that the central nervous system is a med- 

 ullary tube, the walls of which form two dorsal zones and two 

 ventral zones. Second, that every nerve fiber arises from a single 

 cell only, and is nowhere united with any other cell. Third, that 

 every nerve fiber has a branching termination. Fourth, there are 

 three kinds of nerve fibers : (1) Medullary, which arise from the 

 nerve cells of the central nervous system proper ; of the medul- 

 lary fibers three kinds are distinguished — namely, those which 

 pass out to form the ventral root, those which pass out to form 

 the lateral root, and those which pass as commissures to the oppo- 

 site side of the tube ; there are also medullary fibers which run 



