SENSE ORGANS 155 



dermis, though some are also found in the epidermis (in man 

 only in the dermis). 



363. The nuclei of all sensory elements of the skin of verte- 

 brates lie in the spinal ganglia and homologous ganglia of the 

 cranial nerves. 



364. The sense organs just described are those which are 

 generally distributed over the whole surface of the body. The 

 senses to which they correspond are, in man, touch, cold, 

 warmth and pain. Each of these senses, with the possible 

 exception of pain, has its own set of sensory elements, although 

 the correspondence between sense organs and senses has not 

 yet been completely determined. How far these senses are 

 differentiated in the lower animals is also not known. 



365. In the higher animals there are also deeper lying sense 

 organs, which are located in the sub-cutaneous connective 

 tissue, in the muscles and tendons and even in the mesentery. 

 To these organs are ascribed a sense of weight and a sense of 

 position, or attitude, of the member of the body with regard to 

 the other members of the body. 



ORGANS OF SPECIAL SENSE 



366. Besides the general sense organs described above, we 

 find in all the higher animals special sense organs, which are 

 developed in very limited regions of the body and which are 

 often very complex in structure. These are the organs of 

 taste and smell, which are stimulated chemically; the organs of 

 hearing and equilibration, which are stimulated mechanically, 

 and the organ of sight, which is stimulated by ether vibrations. 



367. From direct evidence we know little about the chem- 

 ical senses of hydra, though as in the case of amoeba we may 

 infer that the choice of food indicates a sense of this kind, but 

 this evidence is by no means conclusive. In the sea anemone, 

 however, it is found by experiment that the tentacles distin- 



