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 
Rosenthal 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. Tli. 2, S. 381. 
