io8 VETERINARY PHYSIOLOGY 



different directions. In the dark the distance of objects is also 

 judged by estimating the extent of movement of the hand and 

 arm necessary to touch them. 



The receptors in deeply seated structures may also be stimu- 

 lated by pressure from without, for Head has found that after 

 the cutaneous nerves to a part have been divided, pressure 

 with the point of a pencil is still felt and well localised. 



C. EXTERO-CEPTIVE MECHANISM 

 The Special Senses 



As we have already seen, 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. 



We have further seen that these may be divided into 

 receptors, stimulated by changes set up close to the body, non- 

 distance receptors and receptors acted upon by changes which 

 may originate at a distance distance receptors. Among the 

 former may be classed the receptors in the skin and mouth, 

 and among the latter those in the nose, ear, and eye. 



NON-DISTANCE RECEPTORS 



(a) FOR CONTACT 

 Tactile Sense 



Even after the nerves from a portion of skin have been cut, 

 contact with the point of a pencil is felt, but a prick with a 

 sharp pin, heat and cold, and such gentle contact as touching a 

 hair are no longer felt, and, if a fold of skin is pinched up, 

 and a pencil pressed on the side of the fold, it is not felt. 

 This means that deeply-placed receptors, the fibres of which 

 travel with motor nerves, can be stimulated by pressure 

 through the skin. This, however, is not the mechanism con- 

 cerned in true tactile sense. 



1. Receptors 



Tactile corpuscles consist of a naked branching varicose ter- 

 mination of axons surrounded by sheaths of fibrous tissue, 

 situated in the papillae of the true skin (fig. 49). 



