196 



Basic Structure of Vertebrates 



Fig. 167. Receptor cells. (A) Olfactory, from the nasal epithelium. (B) Gusta- 

 tory, from a taste-bud. (C) Auditory, a "hair cell" from the organ of Corti. 

 (D) Visual, a cone cell of the retina. 



The gustatory cell bears, at its distal end, a single stiff "bristle"; the olfactory 

 and auditory cells bear, distally, numerous fine "hairs," in appearance resembling 

 cilia. 



The arrows indicate the direction of the incoming stimulating agency. The 

 retinal cell is "inverted" in relation to the impact of incoming light. 



(N) Nucleus; (N a ) fiber of (afferent) olfactory nerve I; (N b ) afferent fiber of 

 cranial nerve VII, IX, or X; (N c ) fiber of (afferent) cranial nerve VIII; (N d ) den- 

 drites of a retinal bipolar cell, a neuron interpolated in the afferent pathway from 

 the receptor to the optic ganglionic layer of the retina. 



The olfactory fiber is an outgrowth of the olfactory receptor. The gustatory, 

 auditory, and visual fibers are outgrowths of cells situated in cranial ganglions, 

 and have merely superficial contact with their receptors. 



Gustatory Organs. The individual organ, known as a "taste- 

 bud," is an ovoid cluster of columnar epithelial cells, each of which 

 bears at its free end a delicate bristle or "hair." The ends of the several 

 sense-cells converge around a small pore through which the fluid from 

 the oral cavity gains access to the sense-cells. Terminal fibrils of a 

 nerve (cranial VII, IX, or X) are closely related to the sense-cells 

 (Fig. 168). The cells of the epithelium adjacent to the sense-cells are 

 elongated and arranged to form a supporting and protecting rampart 

 about the taste-bud. 



- Taste-buds occur usually only in the mouth and, to some extent, 

 in the pharynx. In fishes they are usually widely distributed in the 



