Section' I. 



iNTRODOCTORf, 



Th3 student of mammaliaa neurology has his attention fix- 

 ed on a mechanisTi of surpassing complexity. In the pursuit of 

 his /jork, he is continually touching problems, both morpholog- 

 ical and physiological, .vhich frequently transcend all his pow- 

 ers. But the complex nervous skein which he seeks to unravel 

 is merely the final -nember of a series reaching backwari through 

 ever simpler and simpler conlitions to the organization of the 

 primitive vertebrate. In other words, the mammalian brain is 

 the product of endless modifications wrought in the original 

 plan of structure by the continual adjustment of nervous mech- 

 anisms to the play of a shifting environment. What the archi- 

 tecture of the ancestral vertebrate nervous system may have 

 been we can never hope to know from actual observation. Port- 

 unately, there are simple vertebrates existing to-day which 

 retain many features of primitive nervous organization. To 

 such animals, the student of neurology must ever turn for the 

 solution of the problems which vex him in higher fields. One 

 of these simple vertebrates is represents! by a selachian of 



the moderi seas, somewhat specialized in certain directions, 

 (3) 



