THE NERVOUS SYSTEM 



261 



If any cardiac nervous elements arise in situ, they certainly 

 remain undifferentiated until those that have a ganglionic origin 

 have already entered the heart. 



The Cranial Nerves. The nerves of the head exhibit a much 

 greater degree of heteronomy than the spinal nerves, and, in 

 spite of much study, knowledge of their embryonic development 

 is still in a very unsatisfactory condition. The same principles, 

 however, apply to the development of both cranial and spinal 

 nerves; the axones of the former like those of the latter arise 

 either from medullary or ganglionic neuroblasts which are re- 

 spectively unipolar and bipolar; but the cranial ganglionic and 



Fig. 154. — Diagram of the relations of the 

 parts of the sympathetic nervous system 

 as seen in the cross-section. (After His, 



Jr.) 

 M., mesentery. Msn., Mesonephros. 

 Other abbreviations same as Fig. 153. 



medullary nerve-nuclei are not similarly segmented, as in the 

 case of the spinal nerves, and hence the axones are not related 

 as dorsal and ventral roots of single nerve trunks; nor has the 

 attempt to interpret the cranial nerves as homologues of dorsal 

 and ventral roots respectively been successful in the case of the 

 most important nerves. Moreover, the olfactory and optic nerves 

 differ from the spinal type even more fundamentally. The olfac- 

 tory is a sensory nerve that arises apparently from the olfactory 



