CENTERS OF CORRELATION. 223 



highly speciahzed than in cyclostomes. Other parts of the brain, 

 however, as the olfactory centers in selachians and the gustatory 

 centers in some bony fishes, are very largely developed and appear 

 more complex than in other fishes. The cerebellum also is 

 extremely large in many selachians and bony fishes. The study 

 of these structures in those forms in which they are highly developed 

 has led to some results which could not be so well attained from 

 the study of any other forms. In bony fishes the development 

 of special centers from the indifferent gray substance has gone 

 farther than in other lower vertebrates. There has been a collect- 

 ing and sorting of elements which are more diffusely placed in 

 ganoids and selachians. This fact makes the brain of the teleost 

 an especially rich field for the study of the centers and fiber tracts 

 constituting the apparatus for the performance of specific functions. 

 Since the study of less specialized brains has given the fundamental 

 plan of structure of the vertebrate brain, the brain of the teleost 

 should now be subjected to careful and detailed study in order to 

 determine the early form of the central apparatus which directs 

 knovm activities. Herrick has just done this for the gustatory 

 apparatus,*and it is much to be desired that the centers and fiber 

 paths involved in other functions should be worked out in the same 

 way. Generally speaking, any vertebrate in which any system 

 of organs is unusually highly developed presents special opportu- 

 nities for the study of the central apparatus of that system and also 

 of the process and method of brain differentiation in general. 



Although much remains to be done in the way of rendering 

 our knowledge of the special centers and their relation to the 

 main functional divisions complete and exact, the description 

 of these centers will be given as far as possible in the form of an 

 account of the functional system of neurones of which they form 

 parts. 



First there are to be mentioned a number of neurones which 

 seem to be a vestige of invertebrate structures which are quite 

 lost in higher vertebrates. These are the Miillerian cells and 

 fibers of cyclostomes and the cells of Mauthner of fishes and 

 amphibia. There are in the brain of Petromyzon over twenty 

 gigantic cells lying in the somatic and visceral motor columns 



