EIGHTH LECTURE. 



THE PRIMARY SEGMENTATION OF THE VERTE- 

 BRATE HEAD. 



WILLIAM A. LOCY. 

 (LAKE FOREST, ILL.) 



THE vertebrate head is the most complex piece of animal 

 architecture with which anatomists have to deal. It has been 

 produced by gradual modifications of a simpler basis, and dif- 

 ferentiation has been carried farther in it than in any other 

 part of the animal. It represents, therefore, the widest depar- 

 ture from archetypal conditions. 



Its complex structure is correlated with the highest grade of 

 functions. The cranial sense-organs and the brain exhibit 

 the highest manifestations of vital activity to be found in the 

 whole range of living structures. The cranial sense-organs 

 serve to bring the organism into relation with the external 

 world, and the brain, besides being concerned with perception 

 and general mental life, contains nerve-centers for the coordina- 

 tion of the vital actions of the body. The structure and de- 

 velopment of the vertebrate head is therefore a topic of unusual 

 interest and importance in comparative anatomy. 



In their efforts to understand the head, morphologists 

 and physiologists have made use of every available means of 

 research ; observations on the minute structure have been 

 supplemented by studies in embryological development and 

 physiological experimentation. The whole work has been 

 carried on from the standpoint of comparative anatomy. The 

 results have not been just what might have been expected, 

 They have not led to the solution of the fundamental questions, 

 but have served rather to open the field and reveal its extent 



