NERVE 99 



specialised receptors, resembling in some respects those 

 of the skin. 



The sensation of hunger is associated with rhythmic 

 periodic contraction of the empty stomach, and it also is due 

 to stimulation of the ingoing nerves in the muscular wall of 

 the viscus. 



The sensation of thirst, although it is due to deficient 

 water throughout the body generally, is experienced in 

 the mouth and throat, which are not moistened by 

 the free secretion of saliva. In these resfions alone 

 does the deficiency of water stimulate any receptive 

 mechanism. 



The outside of the body is richly supplied with a variety 

 of receptors, each one of which has a low threshold of 

 stimulation for one particular kind of stimulus and a very 

 high threshold for all other kinds. 



Those developed in connection with the skin are generally 

 stimulated by changes in the immediate vicinity, while 

 those developed in the head, such as the eye, ear, and 

 olfactory receptors, are stimulated by changes at a 

 distance, thus warning the animal of conditions it is 

 approaching, 



II. Cutaneous. 



A. Structure. — The cutaneous receptors are of three 

 kinds : — 



1. Dendritic terminations of nerves between the cells of 

 the deep layer of epidermis. 



2. Tactile corpuscles, consisting of a naked branching 

 varicose dendrites enclosed in a fibrous capsule (fig. 43). 



3. A plexus-like arrangement of the dendrites of nerve 

 fibres round the roots of the hairs. 



B. Physiology. — When the sole of the foot is stroked, 

 the leg is involuntarily, i.e. reflexly, drawn up. If the skin 

 •of the foot is subjected to a high temperature, it is 

 withdrawn. These are examples of simple responses to 

 stimulation of the receptors in the skin, and these responses 

 are frequently accompanied by sensations of a definite kind. 



