GENERAL ORGANOLOGY 



115 



experience; following out this course of thought has led to the idea of a 

 'sixth sense,' a designation which is no longer correct since mankind has 

 more than five senses. The former sense of 'touch' really includes, 

 besides true touch, the senses of temperature and pain. In addition there 

 are also the muscular and equilibrium senses. A still more important 

 reason for our very fragmentary knowledge of animal sensations is the 

 fact that, in regard to the function of the sensory apparatus, we can 

 seldom depend upon experiments, and consequently must base our 

 conclusions upon structure. But the anatomy of many sensory organs, 

 like those of smell and taste, is by no means so characteristic that it alone 

 is sufficient to determine the function. 



Fig. 8o. Fig. 8i. 



Fig. 8o. — Tactile hairs of a crab Cyrtomaia (after Dollcin). 



Fig. 8i. — ^\'ater-Pacinian corpuscle of the mesentery of a cat. a, a.xis cj'lincler; /, 

 fat; g, blood-vessel; i, inner bulb; k, capsule with nuclei; «, meduUated nerve-fibre. 



Tactile Organs. — The skin is tactile, usually over the whole area, 

 although not everywhere with equal intensity. Prominent parts, like 

 the tentacles of polyps and of many worms, the anteniicc of arthropods and 

 snails, need only mention. Special epithelial cells with stiff hairs pro- 

 jecting above the surface, the tactile bristles or tactile hairs, are tactile 

 (fig. 80). Only in the vertebrates do the nerves of touch terminate in 

 specially modified end organs (Vater-Pacinian corpuscles, corpuscles of 

 Meissner, etc., fig. 81); these usually lie under the epithelium. 



Organs of Smell and of Taste are accurately known only in verte- 

 brates. The olfactory organ of fishes consists of two pits in the skin. 



