376 A MANUAL OF VETERINARY PHYSIOLOGY 



In the dog the maximum is reached from six to nine hours after 

 a meal, during which time from 20 to 25 per cent, more heat is 

 produced. In the horse, according to Siedamgrotzky, the 

 temperature as the result of feeding may rise 0-2° to o - 8° C. 

 (0-4° to 1-4° F.) ; but, according to this observer, there is no 

 similar rise in the ox ; and Wooldridge found not more than 

 0-3° F. in dairy cattle. That heat is formed during the masti- 

 catory processes we have already seen from the observations on 

 the masseter muscles of the horse ; but the mechanisms for regu- 

 lating heat in the body are such that a rise of anything like 

 1 -4° F. as the result of feeding must be regarded as exceptional. 



Influence of the Nervous System on Heat Production. — The 

 muscles of the skeleton are not always actively contracting, 

 yet heat is always being formed in them. The heat produced 

 in resting muscles is formed as the result of Muscle tonus — viz. 

 the contracted condition of the muscles essential to posture. 

 There is also in operation, even with the most trifling movement, 

 an antagonism to muscular contraction. For example, the 

 flexors of a limb cannot contract without the extensors being 

 thrown into a condition to oppose the movement. Heat pro- 

 duction in muscles is under the control of the nervous system. 

 If an animal be poisoned with curare the motor end-plates are 

 paralysed, in consequence less heat is formed in the muscles, and 

 the temperature sinks ; in fact, the animal becomes for the time 

 being practically cold-blooded, the body temperature rising and 

 falling with the surrounding temperature. The same condition 

 may be produced by dividing the spinal cord in the neck, by 

 which means the motor nerves are cut off from the muscles, and 

 the animal becomes practically cold-blooded. In chloroform 

 narcosis heat production is also greatly interfered with ; in 

 prolonged operations this should be borne in mind and the loss 

 of heat guarded against. Shivering is a physiological process 

 associated with the production of heat to compensate for a 

 loss. The shivering which occurs with horses after being 

 watered is caused by the consumed water abstracting heat from 

 the tissues in order that its temperature may be raised to that 

 of the body. The ' freshness ' of a horse on a winter's morning 

 is the outcome of nervous impulses instinctively started with the 

 object of generating more heat in the body. 



Heat Centres and Heat Nerves. — Apart from contraction, it is 

 believed that muscles are the seat of a quiescent heat production 

 under the influence of the nervous system, and that chemical 

 changes resulting in the formation of heat are generated as the 

 result of nerve impulses. Experimental injury to the corpus 

 striatum, the so-called ' heat puncture,' causes an increased pro- 

 duction of heat, which may last for some time without, appar- 



