THE HORSE AND ITS DISEASES. 99 



The Nerves. 



The nerves are white masses sent off from the brain, 

 some of which proceed out of holes in the cranium from 

 that viscus itself ; others arise fi'om a large mass called 

 the spinal marrow, which extends down the vertebral 

 canal, and gives off trunks through the vertebral 

 holes. 



The nerves in themselves appear long whitish cords, 

 whose internal structure is fibrous, and those fibrillac 

 are distributed to every part of the body. It is supposed 

 that the brain is the seat of sensation, and that the 

 nerves are only messengers of it to all the parts. 



The sensibility of a part is usually proportioned to the 

 quantity and size of the nerves it possesses ; it is evident 

 therefore, they must exist in far greater plenty in some 

 parts than in others. 



Nervous influence is the occasion of another very 

 important phenomenon, which is that of motion, not 

 only the sensation of a part, but its mobility also, is lost 

 upon dividing the nerves going to it. The mobility of 

 parts brought about by nervous influence, is in some 

 measure dependant on the will, and in others it is inde- 

 pendeut of it. In the one instance it is termed volun- 

 tary motion, and in the other involuntary motion. 



iSTerves are capable of reunion after they have been 

 divided, that is, a substance is interposed between, 

 possessed of the power of carrying on nervous influence. 

 We have no reason to suppose there are any of those 

 diseases termed nervous in the horse ; the only active 

 disease Ave know the nerves of the horse are subject to, 

 is locked jaw, which seems an increase of the action of 

 the brain, or of that action which communicates nervous 

 energy to the muscular fibre — hence pressure on the 

 brain has been found for a short time to relieve it. 



The medullary portion is connected with the nervous 

 system ; the nerves are a prolongation of it, and are con- 

 cerned in the discharge of all the offices of life — thoy 



