390 



CHORDATE ANATOMY 



From each of these cells protoplasmic processes extend in two directions — 

 one towards the central nervous system and one towards the periphery. 

 The olfactory nerve is peculiar in its derivation from cells in the olfactory 

 pit which extend their neurites towards the brain and hence are unipolar. 



The neural crest is formed of cells left between the neural tube and 

 the skin when the neural tube separates from the skin. The crest, which is 

 primarily continuous, becomes secondarily broken up into the series of 

 cranial and spinal ganglia. 



Development of S3nnpathetic Ganglia. The sympathetic ganglia of 

 vertebrates are derived, like the neurilemma cells, from the dorsal (sensory) 

 ganglia by the migration of cells ventrally along the nerves toward 



PLEXUS OF AUERBACH^^ 

 PLtXUS OF UElSSNERIk 



jLONGITUDINAL 



-PERITONEUM 



Fig. 346. — A stereogram of a portion of tlie small intestine, showing the arrangement 

 of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are 

 shown in black, sensory cells with white nuclei. (Redrawn after Kahn.) 



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 sympathetic ganglia the nerve cells 

 "spin" the postganglionic fibers to the blood-vessels and viscera. Con- 

 nexions with the nerves from which the sympathetic ganglia arise appear 

 as rami commimicantes. The metameric ganglia become secondarily 

 connected by a sympathetic cord which runs parallel with the dorsal aorta. 

 The prevertebral plexuses, cardiac, celiac, and hypogastric, arise by the 

 more extensive migration of cells from the ganglia of the vagus nerve. 



