THE RECEPTORS AND EFFECTORS 



91 



B. SPECIAL VISCERAL? GROUP. 



22. Organs of Taste. These are excited by chemical stimulation of 

 taste-buds on the tongue and pharynx by sweet, sour, salty, or bitter sub- 

 stances. In man this is a strictly interoceptive sense; but in some fishes 

 taste-buds are scattered over the outer body surface in addition to the mouth 

 cavity, and thus may serve as exteroceptors also. The organ is a flask- 

 shaped collection of specialized epithelial cells of two sorts, supporting and 

 specific sensory elements (Fig. 35). There is a double innervation, partly 

 by perigemmal fibers whose endings surround the bud, and partly by intra- 

 gemmal fibers which penetrate the bud and arborize in intimate relation 

 with the specific sensory cells. 



23. Organs of Smell. These are excited by chemical stimulation of the 

 specific olfactory mucous membrane of the nose. The number of substances 



Fig. 35. Taste-bud from the side wall of a circumvallate papilla of the 

 tongue: a, Taste-pore; b, nerve-fibers, some of which enter the taste-bud 

 (intragemmal fibers), while others end freely in the surrounding epithelium 

 (perigemmal fibers). (After Merkel-Henle.) 



which may act as stimuli is greater than in the case of taste-buds, the num- 

 ber of subjective qualities is also greater, and the discrimination threshold 

 is much lower (see pp. 75 and 218). The peripheral organ of smell is a 

 specific sensory epithelium within the nose, whose sensory cells give rise 

 directly to the fibers of the olfactory nerve, this being the only peripheral 

 nerve of the human body whose fibers arise from superficially placed cell 

 bodies (Fig. 36). 



That the olfactory system was originally an interoceptive sense seems 

 clear; but in all vertebrates living at the present time, the visceral responses 

 to smell are less important than the somatic reactions. The sense of smell 

 is the leading exteroceptor in most lower vertebrates, and this function has 

 been secondarily derived from the primary visceral function. We have seen 

 above that the sense of taste in some fishes has secondarily acquired extero- 

 ceptive functions; and in the case of smell this secondary change has been 

 carried still further until the exteroceptive function has come to dominate 



