840 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



class of sensations the term subjective is given in contra-distinction 

 to objective sensations, which result from impressions made upon 

 the termination of the nerve. It is a curious fact with regard to 

 internal or subjective sensations that those which arise from impressions 

 made upon a nerve in its course are always referred to the peripheral 

 termination of the nerve on the surface of the body. Thus, if the ulnar 

 nerve is compressed at the elbow-joint the impression is not felt at 

 the point of stimulation, but at the extremity of the fingers. The most 

 common cause of subjective sensations is to be found in changes of the 

 blood-supply of the part; thus, for example, when congestion takes 

 place about the termination of the optic nerve there are flashes of light 

 about the eye; if about the auditory nerve, ringing sounds in the ear. 



By the term sensory organs is meant those parts of the body by 

 which, through the nerves of sense, the brain becomes cognizant of its 

 surroundings. 



By means of the special senses the preservation of both the indi- 

 vidual and of the species is rendered possible. By means of the special 

 senses animals are rendered capable of seeking and recognizing their 

 food and are enabled to avoid danger, and, in a way to be indicated 

 directly, are led to the accomplishment of the act of reproduction. 



The more restricted the peripheral portion of any nerve of special 

 sense the more delicate is the structure of that terminal organ. In the 

 case of the sensations of taste and of smell the nerves distribute them- 

 selves over a moist mucous membrane which has other functions to fulfill 

 in addition to acting as the peripheral terminations of nerves of special 

 sense. On the other hand, in the case of the nerves of sight and hearing, 

 the terminations are found in structures whose sole function is found in 

 ministering to these special sensations, and in them we find the highest 

 degree of complexity of the end apparatuses. 



It is, of course, not possible to decide whether or not in the case of 

 the lower animals impressions made upon the nerves of sense affect the 

 brain in the same manner as in man, since in these animals the expression 

 of sensations is greatly restricted : but from the great similarity of struc- 

 ture of the organs of sense in man and in the higher mammals it may 

 be concluded that impressions are appreciated by these animals in the 

 same manner as in man. As a general rule it may be stated that the 

 senses of the domestic animals are quite as highly developed as in man, 

 and in certain instances greatly exceed in acuteness the corresponding 

 sense in man. Usually, in any special class of animals we find one of 

 the special senses developed out of proportion to the others. Domesti- 

 cation has the usual result of reducing the acuteness of the special 

 senses, since the principal cause for their exercise in the protection of 

 the animal is no longer present. 



