i 7 8 



COMPARATIVE MORPHOLOGY OF , VERTEBRATES. 



in character and the organs consequently differ in structure according 

 to the impressions they are to receive. 



With very few exceptions the characteristic portions of the organs, 

 the sensory cells, arise from the ectoderm, but accessory parts, chiefly 

 of mesodermal origin, may be so abundant as to form the bulk of the 

 organ. In some cases the organs may remain in connexion with the 

 surface of the body (the parent ectoderm) throughout life, but frequently 

 they sink to a deeper position and become surrounded with a protective 

 sense capsule, while those connected with the sympathetic system may 

 be scattered throughout almost the entire body. 



FIG. 175. Free nerve termina- 

 tions in the skin of Salamandra, 

 freely after Retzius. 



FIG. 176. Sensory cells, after Fiirbringsr. 

 a, crista cell of ear; b, rod cell of eye; c, ol- 

 factory cell. 



The recipient structures may be of two kinds. In the one (fig. 

 175) the ends of the nerve receive the impressions from without, often 

 aided by various accessory structures. In the other there are specialized 

 sense cells (fig. 176), the peripheral ends of which bear different kinds 

 of cuticular percipient parts hairs, bristles, rods, cones, etc. while 

 the basal ends of the cells are connected with the terminations of nerve 

 cells which act as the conducting elements. The distinction between 

 the two is one of convenience rather than one of physiological or mor- 

 phological importance, for the 'nerves' of the first are in reality but 

 the prolongations of sensory cells. 



Nerve-end Apparatus . 



In many cases skin, alimentary tract, muscles, etc. the ends of 

 the sensory nerves lose their medullary sheath and break up into fine 

 fibrillae which terminate, without special accessory structures, among the 

 cells of the tissue to which they are distributed (free nerve termina- 

 tions). On the other hand, there are numerous end organs, espe- 



