208 NERVOUS SYSTEM OF VERTEBRATES. 



It seems to be formed in part from the ganglionic anlage known 

 as the Nervus thalamicus, and early comes into relation with 

 the ophthalmicus and oculomotorius nerves. In the chick it is 

 derived in part from the neural tube and hi part from the most 

 anterior portion of the ganglion of the ophthalmicus profundus 

 nerve. The development of the other sympathetic ganglia of the 

 head has not been directly followed. Ganglia are present in bony 

 fishes in connection with each of the dorsal cranial nerves, from 

 which presumably they are derived during development. As 

 these nerves have no somatic motor component it is evident that 

 this 'component does not enter into the head sympathetic. It is 

 therefore probable that the fibers which grow out from the ventral 

 spinal nerves to help form the sympathetic are not somatic motor 

 fibers. Indeed, it is known from the adult structure in mammals 

 that the fibers which go by the ventral spinal roots to the sym- 

 pathetic take their origin from cells in the visceral motor column 

 of the cord. In cyclostomes the only connections of the sym- 

 pathetic with the cranial nerves are with the visceral portions of the 

 facialis and vagus. Indeed, from what we know of the develop- 

 ment and structure of the sympathetic system in lower vertebrates, 

 the general conclusion must be drawn that it is an outgrowth 

 from the visceral nerves, including efferent fibers and ganglion 

 cells. The formation of a relatively distinct system is due pri- 

 marily to the migration of ganglion cells along the primitive nerves 

 of the viscera toward the areas supplied by them. 



In describing the structure of the sympathetic system four 

 types of nerve elements must be considered: (i) sensory fibers 

 whose ganglion cells are in the spinal ganglia; (2) efferent fibers 

 whose cells of origin are in the spinal cord or brain; (3) sympathetic 

 excitatory cells; and (4) sympathetic sensory cells. The descrip- 

 tion of these elements will be more clear if Fig. 108 is consulted 

 in connection with the text. 



(i) Sensory fibers whose ganglion cells are in the spinal ganglia. 

 These fibers are the visceral sensory fibers already described in 

 a previous chapter (p. 156). They are the largest of the myelinated 

 fibers running in the sympathetic nerves and may be seen to pass 

 through one or more of the sympathetic ganglia without forming 



