146 THE BRAIN OF THE TIGER SALAMANDER 



CHEMICAL SENSIBILITY 



The peripheral terminals of the sensory fibers of the V to X cranial 

 nerves take three forms: (1) The fibers of the general somatic sensory 

 and visceral systems have free nerve endings within or beneath 

 epithelium or widely spread in deeper tissues. (2) The end-organs of 

 the special somatic sensory systems are differentiated epithelial 

 structures of the internal ear or lateral lines with receptive hair cells, 

 which are shorter than the surrounding supporting cells. (3) The 

 chemoreceptors of the gustatory (special visceral sensory) system 

 are budlike epithelial structures, which resemble the naked lateral- 

 line organs but differ from them in that the specific receptive cells are 

 slender, elongated elements, which span the entire thickness of the 

 epithelium. The fibers which innervate them are generally thinner 

 than those of lateral-line organs and are less myelinated or unmyeli- 

 nated. 



Those species of fishes which have taste buds abundantly distrib- 

 uted in the outer skin and also naked organs of the lateral lines not 

 inclosed in pits or canals present both morphological and physiologi- 

 cal problems of great diflSculty ('03, '03a, h). In the earlier literature 

 all these cutaneous organs were termed indiscriminately "terminal 

 buds," with resulting confusion which was not clarified until the 

 nerve fibers which supply them were found to belong to different 

 functional systems. The fibers supplying lateral-line organs, wherever 

 situated, converge centrally into the acousticolateral area, and fibers 

 supplying taste buds,whether in mucous surfaces or in the outer skin, 

 converge into the f. solitarius and its nucleus. The separation of the 

 gustatory from the lateral-line system of cutaneous sense organs by 

 the anatomical method has been confirmed by physiological experi- 

 ments performed by the writer, G. H. Parker, and others. 



Though this distinction is perfectly clear in some species of fishes, 

 in others there are transitional forms of "terminal buds," and much 

 remains obscure about the functions of these various types of recep- 

 tors. The problem is complicated by the fact that in fishes the skin is 

 everywhere very sensitive to a large variety of chemical substances 

 (Sheldon, '09; Parker, '12, '22; Ariens Kappers, Huber, Crosby, '36, 

 chap, iii; for a more general discussion of the chemical senses see 

 Moncrieff, '44). The skin is sensitive, in general, to different sub- 

 stances from those which activate the olfactory organ and taste 

 buds, but there are some puzzling exceptions. 



For instance, the gurnard fishes (Prionotus, Trigla) have three 



