484 A MANUAL OF VETERINARY PHYSIOLOGY 



The receptors of the labyrinth, and those of the deep receptors of 

 joints and muscles, he shows are stimulated by the animal itself, 

 while the surface receptors (extero-ceptors) are stimulated from 

 without. The deep receptors (proprioceptors of a limb can, how- 

 ever, only influence the tonicity of the muscles of that limb, while 

 those of the labyrinth not only influence the limbs and the trunk, 

 but the head and muscles of the eyeball. In consequence the 

 limb influences are mainly concerned with the relation of the 

 limbs to other portions of the trunk, while the influences from the 

 labyrinth regulate the position of the body generally towards 

 the external world and gravitation. Sherrington regards the 

 chief ganglia of these important functions as being located in 

 the cerebellum. 



The maintenance of equilibrium and sense of direction are 

 associated with movements of the eye muscles, the labyrinth 

 furnishing the necessary information of the relationship of 

 the body to surrounding objects. One of the earliest indica- 

 tions of disease of the spinal cord may be an inability to walk 

 in the dark, and we have seen that a dog without a cerebellum 

 cannot move with the eyes covered. The cerebellum is 

 furnished with the needful information through its connection 

 with the third and fourth pairs of cranial nerves, which 

 govern the movements of the eyeball. It would appear that 

 the biped derives more information from perceiving the position 

 of the limbs relatively to the body than is afforded the 

 quadruped, whose eyes are in advance of the limbs. The writer 

 has never known a horse refuse to advance, even on the darkest 

 night, provided the ground be good. If broken, he travels with 

 care ; if boggy, he may refuse to advance. Without seeing his 

 limbs or surrounding objects, he moves as safely in the dark as 

 in the light, while his sense of direction over ground he has 

 previously travelled is so excellent that the man who has lost his 

 way may safely leave the solution of the problem to his horse. 



Each half of the cerebellum controls the muscular system on 

 its own side of the body, and each half receives impulses from 

 the opposite cerebrum. It appears likely that definite regions 

 of the cerebellum govern definite body areas. It can be shown 

 experimentally that a particular region of the cerebellar cortex 

 controls movements of the fore-foot, another governs a hind -foot ; 

 others cause rolling or bending movements of the body, or con- 

 jugate movements of the eyes. The posterior portion of the 

 vermis is concerned mainly in muscular co-ordination, and in 

 some of the lower animals is the only portion of the cerebellum 

 represented. Finally, we have seen that, after experimental 

 removal of the cerebellum, muscular co-ordination may still be 

 established, which suggests that this organ cannot alone be the 

 seat of this function. 



