r5o COMPARATIVE ANATOMY 



glossopharyngeal and vagus nerves are mixed, like the trigeminal, and 

 have a similar double origin. Their sensory elements come from the 

 neural crest and skin, while their motor fibers arise as processes (neurites) 

 of motor cells located in the lateral wall of the medulla. One peculiarity 

 of the V, VII, IX and X nerves is that their ganglia receive cellular 

 increments from the skin just above the visceral pouch with which the 

 nerve is associated. The patches of thickened ectoderm which proliferate 

 these cells are known as epibranchial placodes. They have been inter- 

 preted, perhaps erroneously, as rudiments of sense organs which have 

 degenerated. 



C. NODOSUM\ 

 C. PETROSUI 

 OTIC G 



ACCeSSOPY NERVE 



CERVICAL GANGLIA 

 /AORTA 



Fig. 462. — The autonomic system of a 16 mm. human embryo. The sympathetic 

 trunk is shown in solid black. The intestine is stippled. (Redrawn from Bremer, 

 after Streeter.) 



Development of Sympathetic Ganglia. Before the embryological 

 facts were known, it was assumed that the sympathetic ganglia of verte- 

 brates represent the chain of ventral ganglia of invertebrates. Their 

 origin in vertebrates proves that this homology is incredible. The sym- 

 pathetic ganglia of vertebrates are derived, like the neurolemma cells, 

 from the dorsal ganglia by the migration of neural crest cells ventrally 

 along the nerves toward the dorsal aorta. They first appear as clusters 

 of cells, each cluster connected with the nerve from which it arose, at 

 the level of the aorta. In the head the ciliary, sphenopalatine, otic, 

 and submaxillary ganglia are formed in this way. In the trunk the 

 superior and inferior cervical ganglia, and the series of vertebral and 

 prevertebral ganglia belonging to the sympathetic are derived from the 

 neural crest by the prolonged migration of nerve cells. In the sympa- 

 thetic ganglia the nerve cells "spin" the postganglionic fibers to the blood 



