310 F. L. LAND ACRE 



standing the enormous amount of labor expended on this prob- 

 lem, it can hardly be said that there is any general agreement 

 among workers as to the mode of origin or fundamental rela- 

 tionships of the various cerebral nerves to each other. 



The lack of agreement may be traced to several causes aside 

 from the difficulty inherent in the problem. Chief among these 

 causes must be placed the subordination of the study of the cere- 

 bral nerves to more general problems, such as the metamerism 

 of the vertebrate head and the relation of the head to the trunk. 

 These fundamental problems have been a source of intense inter- 

 est to both morphologists and embryologists and it was only 

 natural that the cerebral nerves should be used as a means of 

 shedding light on them, even when the origin, composition and 

 distribution of the nerves were not accurately known. 



Owing to this method of approach, however, students of 

 the cerebral nerves were for a long time handicapped by pre- 

 conceived notions as to the relations which ought to exist between 

 the trunks of the cerebral nerves and the various head segments. 

 The idea of the serial homology of the cerebral nerves has been 

 in a sense a barrier to a clear understanding of their true rela- 

 tionships. The chief function of the nervous system as a coor- 

 dinating mechanism was lost sight of in the attempt to reduce 

 the cerebral nerves to some fundamental type that would bring 

 them into harmony with each other and with the spinal nerves. 



Some such fundamental relationship among the cerebral 

 nerves as that involved in the idea of serial homology probably 

 existed in a primitive vertebrate; but the cerebral nerves of the 

 Ichthyopsida are not most easily understood on such a basis. 

 The fundamental needs of the organism have so modified this 

 primitive type that it is much easier and more in accord with 

 the facts to take some functional unit, such as a sensory system, 

 as a basis for the analysis of the cerebral nerves. 



The fundamental difficulty in the attempt to homologize the 

 trunks of the cerebral nerves comes from the fact that they are 

 not units but composite in nature, so that the components of 

 which they are made up furnish the logical basis for getting at 

 their true relationships. 



