188 VETERINARY PHYSIOLOGY 



the body dilate, the arterial blood pressure falls, and thus 

 less blood is sent to the brain, and the organ becomes more 

 bloodless. This may be seen in cases of trephining, where a 

 sinking of the trephine scar occurs during sleep. (3) The 

 eyelids close, (4) the eyeballs turn upwards, (5) the pupils 

 contract, and (6) the voluntary muscles may relax. In the 

 horse, ox, etc., the tonic contraction of the muscles, with the 

 assistance of the ligaments (p. 237) sustaining the body 

 in the standing position, may persist, and the animal may 

 sleep in that position. 



The depth of sleep may be measured by the strength of 

 the stimuli required to overcome it. 



The prejudicial effect of want of sleep has been demon- 

 strated by observations upon young dogs. It was found that 

 they died in five days if prevented from sleeping, while, 

 if allowed to sleep, they withstood even the withdrawal of 

 food for no less than twenty days. 



More sleep is required by young than by old animals. 



8. Hypnosis. 



This is a condition in some respects allied to sleep. In 

 many of the lower animals it is readily produced, simply by 

 holding the animal firmly and placing it in any unusual 

 position, as on the back, especially if stimuli from without 

 are cut off— e.gr. by bandaging the eyes. It may then lie with- 

 out moving for a prolonged period, and may react much in the 

 same way as if decerebrated. The condition may be induced 

 in many people by powerfully arresting the attention, and it is 

 probably due to a removal of the influence of the higher 

 centres over the lower. The respirations and pulse become 

 quickened, the pupils dilate, and the sensitiveness of the 

 neuro - muscular mechanism is so increased that merely 

 stroking a group of muscles may throw them into firm 

 contraction. This suggests an abrogation of the cerebral 

 function and a dominance of the tonic vestibulo-cerebellor- 

 spinal arc. The individual becomes a reflex machine even as 

 regards the cerebral arcs, and each stimulus is followed by an 

 immediate reaction. 



