164 



PRINCIPLES OF ANIMAL BIOLOGY 



there must be appropriate sensory endings and nerve cells. The types 

 of endings responsible for the mediation of the different classes of sensa- 

 tions cannot be distinguished on the grounds of histological structure 

 but only by experiment. Figure 122 illustrates the termination of an 

 afferent fiber in the skin of the salamander. 



Organs of Special Sense. — The sensations of light, sound, taste and 

 smell are not perceived through scattered sensory cells but through the 

 grouped sensory cells of the organs of special sense, the eyes, ears, taste 

 buds and olfactory epithelium respectively. Although the structure 

 of these organs is very different the sensitive cells of each may be con- 



^... Hbr.Schi 



..Ml.La 



'i'Y"-'Ba.Schi 



Fig. 122. — Termination of afferent fibers in the skin of the salamander, Salam- 

 andra maculosa. Ba. Schi, basal layer of skin; Cor, corium ; Hor. Schi, stratum corn- 

 cum; Ml. La, middle layer; nf, nerve fibers; x, end of a terminal. {Froin Schneider after 

 Rctzius.) 



sidered to be modified ectodermal epithelial cells connected in some 

 manner with the central nervous system. The sensory cells of the ear 

 and of the retina of the eye and of the taste buds are connected with the 

 central nervous system by means of nerve cells whose peripheral branches 

 are in contact with the bodies of the sensory cells and whose inner branches 

 terminate in the central nervous system. The cells of the olfactory 

 epithelium, on the contrary, have no intercommunicating cells between 

 them and the central nervous system but each cell has a fine process 

 which extends from the base of the cell to the central nervous system. 



References 



Howell, W. H. Textbook of Physiology. 



Martin, H. N. The Human Body. 



Skerrixgton. C. S. The Intocrative Action of the Nervous Svstem. 



