ANIMAL HEAT. 697 



neous secretion, and, as a consequence, the greater will be the loss of 

 heat through evaporation. Everything, therefore, which dilates the 

 cutaneous blood-vessels will increase the heat dissipation. The working 

 of the mechanism of heat regulation through increasing heat dissipation 

 is well seen in the case of exercise. Every muscular contraction, as 

 already pointed out, leads to an increase of heat production, and, as a 

 consequence, the blood coming from a contracting muscle may be several 

 degrees warmer than the arterial blood supplied to it. Nevertheless, the 

 body temperature, even in severe exercise, is but little elevated above the 

 average, for the increased exercise leads to an increased demand of 

 oxygen in the inspired air, and, as a consequence, respiration is increased 

 and the amount of heat eliminated through the expired air thereby aug- 

 mented. The action of the heart is likewise accelerated, the circulation 

 through the skin is augmented, perspiration is increased, and a greater 

 amount of heat is given up from the skin by radiation and by the evapo- 

 ration of the perspiration. By this means enough heat is lost to the 

 animal body to balance the increased production. 



Increase of external temperature likewise prevents an increase in 

 the body temperature by increasing the circulation through the skin 

 and the cutaneous perspiration, and so also increases the loss of heat. 

 On the other hand, if the external temperature is reduced the cutaneous 

 vessels contract, the evaporation of perspiration is prevented, and again 

 a balance is struck between the production and the heat dissipation. 



The influence of the nervous system on the temperature of the 

 animal body is both directly and indirectly exerted. It has been men- 

 tioned in a previous section that division of the cervical sympathetic 

 nerve is followed not only bj' contraction of the pupil, but also by an 

 increased temperature of the corresponding side of the head and neck. 

 If this experiment be performed upon a rabbit the increase of tempera- 

 ture is so great as to be readily perceptible to the touch ; and if the ears 

 of the rabbit be examined in transmitted light the blood-vessels of the 

 auricle on the side of section of the sympathetic will be found to be 

 greatly dilated. Section of the sympathetic, therefore, by paralysis of 

 the vaso-motor nerves r has occasioned dilatation of the blood-vessels, and 

 the consequent increased vascularity facilitates radiation of heat and so 

 causes a perceptible increase in the temperature of the part. But the 

 increased radiation of heat is also attended with increased heat produc- 

 tion; for the hypersemia produced by vaso-motor paralysis is accom- 

 panied by increased nutritive activity, and heat production is thereby 

 augmented. Thus, it has been shown by Bidder that excision of a part 

 of the cervical sympathetic in the rabbit is followed within two weeks by 

 a marked increase in the size of the ear on the side of operation ; and 

 excision of the cceliac plexus has been said to produce intense hypersemia 



