VII I THE FUNCTIONS OF THE SKKST 91 



converse happens when the temperature is cold and the body is 

 at rest; then the surface vessels become constricted and the 

 heat loss is greatly reduced. 



If however the external temperature is very high or exercise 

 is severe no amount of dilatation of vessels is sufficient to produce 

 the necessary heat loss, and then another regulating mechanism 

 comes into play, and this is the secretion of sweat. The moisture 

 generally evaporates quickly unless formed too fast. It is a 

 means of losing bodily heat. The secreting process is essentially 

 a nerve reflex, and as in the case of the salivary glands may be 

 antagonised by administering atropine. Moreover, by stimulating 

 certain nerves electrically, sweat secretion can be increased (e.g. 

 by stimulating the sciatic nerve in the dog when the sweat 

 glands of the foot-pad secrete profusely) . 



There is experimental evidence of the existence of a centre 

 in the fore brain which presides over and coordinates all the 

 functional activities concerned with both heat production and 

 heat loss. 



As already mentioned there is a compensating action between 

 sweating and the secretion of urine, these two processes being 

 inversely proportional in their respective activities. Further- 

 more, the sweat glands besides supplying a heat -regulating 

 mechanism are definite organs of excretion. 



The horse is the only hairy animal which sweats easily from 

 nearly every part of the bod3^ A horse begins to perspire at 

 the bases of the ears, the neck, chest and back follow, and finally 

 the hindquarters. Sweating does not take place on the legs. 

 With donkeys and mules sweating is confined chiefly to the bases 

 of the ears. Oxen sweat mainly on the muzzle, and only with 

 some difficulty elsewhere. Sheep also perspire very little, the 

 number of sweat glands being relatively few. In pigs sweating 

 only takes place on the snout. Dogs and cats can sweat profusely 

 on the muzzle and foot-pads, but not on the general surface of 

 the body. A dog when heated by exertion pants and throws out 

 its tongue thereby admitting of an increased conduction of heat, 

 but the glands inside the mouth are not sweat glands. 



The fact that oxen and many other animals possess com- 

 paratively few sweat glands and do not perspire freely on the 

 body explains the mi;ch greater range of body temperature 

 which these animals normally possess. They cannot undergo 



