SENSORY ORGANS 



193 



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 fre- 

 quently 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. 



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

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

 aided by various accessory structures. In the other there are special- 

 ized sense cells (fig. 198), the peripheral ends of which bear different 

 kinds of cuticular percipient parts — hairs, bristles, rods, cones, etc. — 



Fig. 197. — Free nerve termina- 

 tions in the skin of Salamandra, 

 freely after Retzius. 



Fig. 198. — Sensory cells, after FUrbringer. 

 A, crista cell of ear; B, rod cell of eye; C, ol- 

 factory cell. 



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 morphological importance, for the 'nerves' of the first are in 

 reahty 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 ter- 

 minations). On the other hand, there are numerous end organs, 

 especially among the terrestrial vertebrates, in which accessory parts 

 are present. For details of these reference must be made to histolo- 

 gical text-books; only a mention of some of the kinds can be made 

 here. 



In the simple tactile corpuscle the nerve terminates with a cup 

 13 



