854 ANIMAL HEAT. 



Rubner calculates that the tissues of a rat produce five and one- 

 third times, the tissues of a sparrow thirteen times, as much heat as the 

 same weight of tissue in a man. 



THE INFLUENCE OF THE NERVOUS SYSTEM UPON THE 

 EEGULATION OF TEMPERATURE. 



The nervous system exercises a control on both of the factors 

 concerned in the regulation of temperature ; upon the loss of heat 

 by means of the vasomotor system, which regulates the amount of 

 blood in the deep and superficial parts of the body, and by the respira- 

 tory centre which controls the frequency and depth of respiration ; upon 

 the production of heat through the nerves which control the activity 

 of the tissues, chiefly the muscles. The control is of the nature of 

 a reflex, and the sensory nerves of the skin and muscles are probably 

 the most usual lines of the afferent impulses. The most important 

 nervous centres are the vasomotor and the respiratory, but in addition 

 to these and the so-called "motor" centres some physiologists maintain 

 that special " heat centres " exist in the brain. 



Vasomotor control of temperature. The blood distributed to the 

 body comes from the heart, where the temperature is, with the excep- 

 tion of the liver and a few other internal parts, the highest in the body ; 

 this warm blood is carried to the extremities and the surface of the 

 body, where the temperature is lower. Now, three zones may, as 

 Bosenthal l has pointed out, be recognised an internal warm zone, an 

 intermediate temperate zone, and an external cool zone ; the first is 

 represented by the deep organs and tissues, the second by the more 

 superficial parts, and the third by the skin and subcutaneous tissue. 

 Under ordinary circumstances the temperature will decrease from 

 within outwards, for the most important seats of chemical change and 

 heat production are situated within the first two zones, and the loss of 

 heat is greatest from the surface of the skin. The blood circulating in 

 the vessels distributes the warm blood of the interior to the superficial 

 parts, and carries back cooler blood from the surface to the interior. 

 The difference, therefore, in temperature between the interior and the 

 surface will depend upon the rapidity and the quantity of the blood 

 circulating through the different zones of the body ; this distribution is 

 regulated by the central nervous system through the vaso-constrictor 

 and vaso-dilator nerves. The vasomotor nerves have their centre in 

 the medulla oblongata, and probably subordinate ones in the spinal cord ; 

 the distribution, however, of these centres and nerves is discussed else- 

 where; here they will be considered merely as part of the nervous 

 mechanism which regulates temperature. 



When the cutaneous and subcutaneous vessels are constricted, the 

 quantity of blood distributed to the skin is diminished, the difference 

 between the temperature of the surface of the body and its surroundings 

 is less, and consequently less heat is lost. This condition is brought 

 about by external cold, and thus the heat of the body is economised and 

 its normal temperature is maintained, or may, under certain circum- 

 stances, be raised, for it has already been shown that the first effect of a 

 cold bath is to raise the temperature in the axilla and rectum. On the 

 other hand, exposure to warmth causes a dilatation of the cutaneous 



1 Hermann's "Handbuch," 1882, Bd. iv. Th. 2, S. 381. 



