36 



ANATOMY OF THE CENTEAL NEEVOUS SYSTEM. 



seen in Fig. 11. But all the cells are only slightly dependent on each 

 other, being, for a large part of their function, wholly independent. As an 

 example of such isolated reflex action, we have the movements occurring in 

 the musculature of a portion of the intestines removed from the body, which 

 take place with entire regularity, when irritated on its mucous membrane. 

 The influence on these short reflex-arcs, which larger and more extensive 

 arcs have, is well known; such, for example, as those passing through the 

 sympathetic ganglia and the spinal nerve-roots. These form new neurons, 

 which are connected with those of the intestines, influencing, exciting, and 

 restraining them. 



THE PEEIPHEEAL NEEVES. 



All examinations of the vertebrates have shown that the motor nerve 

 arises from a large ganglion-cell, which sends out its neuraxon to a muscle. 



jRat&cani. 



Fig. 13. — Section of the spinal cord of a human embryo of the fourth week. 

 Note ventrally the anterior or motor root developing from cells of the cord. In 

 the dorsal portion (four and one-half weeks) the sensory root grows in from cells 

 of the spinal ganglion. (Combined from figures by His.) 



where it ends by division. Everything that has been observed with in- 

 vertebrates indicates that there, also, this is the case. On the other hand, 

 with invertebrates the direct obser-\'ation has been made of nerve-fibers 

 originating in cells in the skin, sensory nerves, passing into the central 

 organ, and there ending by free extremities. His has made valuable ex- 

 aminations along this line with vertebrates (Fig. 13). 



In embryos of vertebrates the central nervous system, as is well known, 

 presents, in the early stages of development, a canal. His determined that 



