THE NERVOUS SYSTEM 



5o; 



the arm and thigh. The muscles themselves are extremely 

 powerful, and possess in their substance a large amount of 

 tendinous material, such as is evident in all muscles constantly 

 in action ; for example, those of the back, masseter muscles, and 

 others. During sleep the horse is unconscious, and the muscles 

 under no kind of cerebral control, yet they remain strong and in 

 operation without the knowledge of the animal. This is due to 

 the reflex actions of the spinal cord, which, as has previously 

 been urged, are so evident in the herbivora (p. 445) ; likewise the 

 reflex control involving the cerebellum is still in full operation 

 in maintaining posture, so that, asleep or awake, the tonic con- 

 dition of the muscles is maintained through their agency, not 

 only of the muscles of the limbs, but of those of the back and 

 chest wall, which are equally essential to the maintenance of 

 the erect attitude. The attitude of the sleeping bird, on one 

 leg, is a similar instance of the maintenance of reflex balance 

 when consciousness is in abeyance. 



The rest obtained by sleeping in the erect attitude is insuf- 

 ficient for hard-worked horses. They need the complete re- 

 laxation of their muscles, and this can only be furnished in the 

 recumbent position. When from anchylosis of the vertebrae or 

 other causes the horse does not lie down, he must be placed in 

 slings at night in order to obtain the needful rest. If not hard 

 worked, they may stand for years without this aid ; on board a 

 ship, or for surgical reasons, they may be kept in a confined 

 position for a considerable period without lying down, if only the 

 muscles be exercised for a few minutes each day. Horses are 

 not free from falling when asleep, they may even actually come 

 to the ground, but this is rare ; what generally happens is that 

 relaxation of the extensors of the fore-limbs occurs, and the 

 animal knuckles over on to the fetlocks and at once recovers 

 itself, but not without inflicting an injury to the skin over the 

 joint. The fall always occurs in front, and not behind ; probably 

 the extra weight on the fore-legs may have something to do 

 with this. 



Observations made on man, and those animals which lend 

 themselves to inquiry, show that during sleep the respirations 

 become slower and deeper, and this can readily be observed in 

 the horse. The secretion of urine becomes lessened, the pulse- 

 rate and blood-pressure fall, and the brain shrinks, so that 

 the volume of blood in the other parts of the body is slightly 

 increased, as might be expected from the general inaction of the 

 muscles. The production of carbon dioxide is lessened, while 

 the loss of nervous control over the production of heat causes a 

 fall in temperature. The essential physiological factor in sleep 

 appears to be the anaemia of the brain. 



