THE VISCERAL AFFERENT DIVISION. 173 



be supposed that the valvula has been formed by a growth and 

 folding inward of the velum of selachians. The cause for the 

 growth of this large structure is to be found in the great increase 

 in the gustatory system in ganoids and bony fishes. 



In selachians a part of the secondary fibers from the visceral 

 lobe cross to the opposite side as do the internal arcuate fibers 

 from the cutaneous nuclei. The tertiary tract from the superior 

 secondary gustatory nucleus goes as in bony fishes to the inferior 

 lobes. Apparently in the more primitive brains the inferior lobes 

 as a whole were related to both gustatory and olfactory systems 

 ( cf. Chap. XVII). The relations in selachians are taken as the 

 basis for the general diagram of visceral sensory structures, Fig- 

 ure 92. Figures 112-117 in Chapter XV may also be consulted. 



The taste buds are evidently unable to persist on the surface 



Secondary gustatory tract 



FIG. 91. Part of a sagittal section of the brain of a newly hatched bony fish, 

 Catostomus, to show the relations of the secondary gustatory tract and the valvula 

 cerebelli. 



of the body in terrestrial animals, and in amphibia, reptiles, 

 birds and mammals they are confined to the mouth cavity. Besides 

 the great modification in structure of the gustatory system which 

 this change entailed, a great change in function has been brought 

 about. In primitive fishes the taste organs in mouth and 

 pharynx detected indications of food brought by the respiratory 

 water current. In fishes where the taste buds are situated on the 

 surface of the body they serve to detect the presence of food, and 



