66 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



In the cranial region the matter is still further complicated 

 by the appearance of a lateralis system, the nerves of which are 

 distributed to the ear and to the lateral line sjstem, and to no 

 other organs. In terrestrial vertebrates, where the lateral line 

 system is lost, the lateralis ner\'es, with the exception of the 

 eighth (auditory), are lacking. The fibres of the lateralis com- 

 ponents terminate in the tuber acusticum. The relations of the 

 sensory components of the cranial nerves are shown in Fig. 6;, 

 in which, for clearness, the motor elements have been omitted. 

 The somatic motor nerves of the head include only the eve- 

 muscle nerves (III., IV., VI.). Visceral motor fibres are found 

 in the fifth, seventh, ninth, and tenth nen'es. 



SEXSE ORGANS. 



All sensory organs of vertebrates arise from the ectoderm. 

 Some remain throughout life connected with the surface of the 



body, — epidermis, — while others 

 sink into special structures for 

 their protection, the sense cap- 

 sules. With few exceptions sense 

 organs are formed of specialized 

 cells, — sense cells, — each of 

 which is connected by afferent 

 nerve fibres with the central ner- 

 vous system. Between the sense 

 cells there may be other ectij- 

 dermal cells which have a sup- 

 porting function, or which serve 

 to isolate the sensory cells from 

 each other. 



These sense organs which are 

 situated in the epidermis are the 

 more generalized, and among 

 them are distributed the sensations of touch, pressure, and tem- 

 perature. In the aquatic ichthyopsida ( Fig. 67) these oro-ans 

 are composed of rod-like, club-formed, or pear-shaped cells, the 

 free extremities of which may reach the surface ; but in all 



Fig. 66. Sense cells, after various 

 authors. .-/, taste cell of rabbit ; B., 

 hair cell from lagena of pigeon ; C, 

 olfactory cell of Proteus; D and E, 

 rod and cone cells from the human 

 eye. 



