NERVOUS SYSTEM 



879 



Fig. 492. 



Transverse section through 

 the body of a typical Vertebrate, 

 showing the peripheral (seg- 

 mental) nervous apparatus. 

 Small dots, afferent visceral 

 neurones ; coarse dots, afferent 

 somatic neurones ; dashes, ef- 

 ferent visceral (ventral root 

 and sympathetic) neurones ; 

 lines, efferent somatic neurones. 

 Darm, gut ; Ggl. spin., spinal 

 ganglion ; Ggl. vert., vertebral 

 sympathetic ganglion ; Ggl. mes- 

 ent., mesenteric sympathetic 

 ganglion. The peripheral sym- 

 pathetic ganglionic plexuses 

 (Auerbach and Meissner) are 

 not shown. Muse., muscle ; Rod. 

 dors., dorsal root ; Rad. vent., 

 ventral root ; R. comm., whit 

 ramus communicans. Two sym- 

 pathetic neurones are repre- 

 sented as intercalated in the 

 visceral efferent pathway. It is 

 doubtful if there should be more 

 than one. (After Froriep.) 



sympathetic fibers, and, as they are non-medul- 

 lated, they are gray in color, so that the trunk 

 carrying these gray fibers from the chain 

 ganglion to the dorsal and ventral branches is 

 called the gray ramus. 



It is important for future work to know 

 that some of the cells from the spinal cord or 

 spinal ganglia of the dorsal root migrate to 

 various parts of the body, and are usually quite 

 closely associated with the glands of internal 

 secretion, such as the hypophysis, carotid 

 gland, suprarenals, etc. These cells have a 

 peculiar affinity for chromic acid salts, and are 

 therefore often called chromaffine cells, though 

 their function is yet unknown. 



THE CRANIAL NERVES 



In the early part of our work, we found 

 that the frog has ten cranial nerves, and that 

 mammals have twelve. The cranial nerves 

 differ from the spinal in not being truly seg- 

 mental nerves (Fig. 493), and in some of them 

 being purely sensory, some purely motor, and 

 still others of a mixed nature. Besides, the 

 cranial nerves which carry sensory fibers all 

 have a ganglion near the root, while those of 

 a pure motor nature do not have. 



They are like the spinal nerves in so far 

 as some of them have somatic sensory, somatic motor, visceral sensory, 

 and visceral motor fibers, but are unlike the spinal nerves in that two 

 additional components occur in connection with the cranial nerves. 

 These are the nerves of special sense, and in the fishes, the nerves of the 

 lateral line (Figs. 340, 479). 



The somatic sensory nerves in the head are called general cutaneous 

 nerves. They terminate in the skin either as free nerve ends or as special 

 sense organs of touch. The visceral sensory fibers end in taste organs, 

 usually inside the mouth, but in some of the teleostomes they are dis- 

 tributed over the surface of the body. The termination of the elements 

 going to make up the lateral line arrangements are sensory, and termi- 

 nate in little collections of sense cells commonly called "hillocks" or 

 neuromasts in the ear and in the lateral line organs of certain groups 

 of fishes and amphibians. 



The lateral line organs have been thoroughly discussed in the em- 

 bryology of the frog. 



