526 



COMPARATIVE ANATOMY 



Striking differences in function and distribution distinguish these 

 twelve and set them apart from spinal nerves. These differences may be 

 briefly summarized: 



I. Olfactorius. The olfactory nerve is peculiar in the origin of its 

 neurites, which, like the neurites of primitive neurosensory cells, develop 

 as processes of sense cells located in the olfactory epithelium. Their 

 terminal arborizations effect synapses with neurons located in the 



ANTERIOR 



PERFORATED 



SUBSTANCE 



OPTIC CHIASMA 



TUBER CINEREUM 



MAMMILLARY BODIES 



LATERAL GENICULATE 

 BODY 



/»_--MASri CATOR NERVE 

 ^ -V 



CEREBELLUM 



'i MEDULLA 

 f OBLONGATA 



DECUSSATION OF PYRAMIDS 



SPINAL CORD 



Fig. 437. 



-Human brain stem, showing nerve connexions, 

 after Allen Thomson.) 



(Redrawn from Morris, 



olfactory lobes of the brain. By means of a neuron chain, connexions 

 are established with the cells in the hippocampus. That the "terminal 

 nerve" of Pincus is an independent segmental nerve has not been demon- 

 strated. That it is a ganglionic division of the olfactory nerve is sup- 

 ported by the evidence of its structure, development and relations. 



n. Opticus. The so-called optic nerve is not a true segmental nerve 

 comparable with other cranial nerves. Its mode of development proves 

 that it is a specialized fiber tract of the forebrain, since the neurosensory 

 cells which form it are originally a part of the brain wall, and their terminal 

 arborizations lie not in the skin but in the retina which develops from the 



