CRANIAL NERVES. 



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of the thalamencephalon is yet more difficult to understand. 

 It bears at its end the pituitary body or hypophysis, part of 

 which is derived from the brain and part from the mouth, 

 (i) It is compared (Balfour, Julin) to the sub-neural gland 

 of Tunicates. (2) It is hypothetically connected (Dohrn) 

 with two abortive gill-slits. (3) It is hypothetically inter- 

 preted (Beard) as a residuum of the original mouth which 

 Vertebrates are supposed to have possessed before the per- 

 sistent one with which we are familiar was evolved, and of 

 the innervation of that hypothetical structure. 



Cranial Nerves. — The origin and distribution of the 

 cranial nerves may be summarised as follows : — 



* The letter J. is a contraction for sensory or afferent, z'.e., transmitting impulses from 

 a sensitive area to the centre ; and vt. is a contraction for motor or efferent, z".e., trans- 

 mitting impulses from the centre to the body. 



