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 
‘ase 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 
