506 A MANUAL OF VETERINARY PHYSIOLOGY 



some room is made for the extra blood by displacement of the 

 cerebro-spinal fluid. 

 Movements of the Brain are dealt with at p. 97. 



Sleep. 



The actual cause of sleep is not known. The tissues 

 require rest and repair, and these are undoubtedly effected 

 during sleep ; but no explanation, which has been generally 

 accepted, has accounted for the loss of consciousness which occurs 

 during the process of anabolism. The amount of sleep required 

 by animals appears to be greater in the carnivora than the her- 

 bivora. The dog and cat spend a considerable time in this con- 

 dition of unconsciousness, but the herbivora sleep much less, 

 and for only short periods at a time. Neither class sleep with 

 the same depth nor intensity as man ; it is not conceivable that 

 a dog or cat could remain in a condition of profound slumber 

 while considerable noise was occurring. In the case of the horse 

 he is such a light sleeper that the faintest footfall suffices to 

 wake him up. He sleeps with his eyes open or semi-open and 

 on his side, fully extended. The ruminant sleeps with the nose 

 turned in to the side, or resting on the chin, with the head extended. 

 The horse sleeps for short periods together ; his immense weight 

 does not admit of his lying for any length of time on one 

 side, for during this period the lower lung does no work and the 

 lower muscles get cramped. He rises and lies on the opposite 

 side, or rolls over to that side ; or may rise, eat for a short time, 

 and again lie down. He requires little sleep, but it should be of 

 good quality. Hard- worked horses cannot do without it. The 

 horse has the power of sleeping while standing, for which the 

 limbs are provided with certain necessary mechanisms in 

 connection with the muscles, which will be examined in the 

 chapter on Locomotion. The horse that sleeps while standing 

 drops the head below the level of the withers, the eyelids partly 

 fall over the eyeball, and the limbs are brought rather more 

 under the body than usual. The extensor muscles of the limb 

 cannot be relaxed, or the body would fall ; they must accordingly 

 continue to have impulses poured into them in order to maintain 

 their contracted condition. Nor is this confined to the limb 

 muscles ; those which sling the body between the fore-legs must 

 also have tone imparted to them if the erect attitude is to, 

 be maintained. The impulses dealing with body equilibrium 

 must also continue in operation. None of the mechanisms of 

 the limbs which aid the muscles and tendons are in any way 

 adequate to explain these facts. The extensor muscles, on 

 which everything depends, receive assistance from the fascia of 



