HISTOLOGY OF THE FROG 



115 



axis cylinder is the first to make its appearance and comes from the ex- 

 terior or ectodermic layer of the organism in which it develops, while 

 the cells forming the sheath come from the mid or mesodermic layer. 



The negative manner of testing any of our scientific laws and prin- 

 ciples may be illustrated here by calling attention to the fact that much 

 of our knowledge of the position of nerves in various parts of the body 

 does not come from our ability actually to trace them throughout their 

 entire course, but by tracing the dying portion of an injured nerve. 

 Having found that the cell is the important part of a nerve, arid that 

 whenever a fiber is cut between its cell and the termination of its 



Vend rites 

 -Cell Body 



.Yode of Bani-ier 

 ,'evrilemma 



Fig. 42. Neurons of Various Types from Higher Animals. . 



A, a complex of neurons from the cerebrum ; B and C, neurons from the 

 cerebellum ; D, a single neuron from the cerebrum. E, diagram of a neuron or 

 nerve unit. 



processes it is that part still attached to the cell which will grow again, 

 experimenters have cut nerve fibers and then watched that portion no 

 longer connected with the cell proper, degenerate. By watching this and 

 then observing those parts of the body that degenerate along with the 

 dying nerve fiber, it is easy to see where the fibers actually pass and 

 terminate. 



Nerve centers is the name given to those parts where several nerve 

 cells are grouped together such as the brain, spinal cord, ganglia, and 

 the various ganglionic masses of the sympathetic system. The centers 

 themselves consist of ganglion cells and their fibers, together with the 

 connective tissue which holds them together, and the little vessels which 

 supply them with nutriment and carry away waste products. 



Ganglion cells are usually quite irregular in outline with a single 

 nucleus near the center. The cytoplasm is rather granular and with 

 certain stains shows a network of tiny fibers connected directly with 

 the fibrillae of the nerve fiber as well as with other processes of the 



