EVOLUTION OF IIEAT. 547 



than the arterial; but in the cavities of the body this law is reversed, as be- 

 tween the crural vein and artery there is a difference of one degree, and very 

 near the bifurcation of the aorta and vena cava, the difference is only eight- 

 tenths of that just mentioned. Immediately above the opening of the renal 

 veins, the blood of the vena cava and aorta have the same temperature, which 

 Bernard terms the real point of temperature. The inferior vena cava's blood 

 is warmer than that in the arteries; that of the superior vena cava is cooler. 



Bernard believes that the vaso-dilators are calorific nerves, whilst the 

 vaso-constrictors are frigorific. 



Prof. H. C. Wood, Jr., believes that in the pons or above it, there is a 

 nerve centre whose function it is to inhibit the production of animal heat. 

 This centre does not act directly upon the tissues, but probably through a 

 controlling influence over centres in the cord, which regulate the chemical 

 activities. The action of the centre is supposed to be analogous to the reflex 

 inhibitory centre of Setschenon.] 1 



438. We have now to inquire whether the power of generating Heat is 

 possessed by the Human subject in an equal degree at all ages; this question 

 being very different from that of the ordinary temperature of the body at the 

 various periods of life; since an individual who can maintain a high tem- 

 perature when the surrounding air is moderately warm, may have very little 

 power of bearing continued exposure to severe cold. Important analogical 

 evidence on this point has been supplied by the experiments of Dr. W. F. 

 Edwards upon the lower Mammalia, Birds, etc. 2 It appears from these to 

 be a general fact, that, the younger the animal, the less is its independent 

 calorifying power. Thus the development of the embryo of all Oviparous 

 animals is entirely dependent upon the amount of external warmth supplied 

 to it. There are many kinds of Birds, which, at the time they issue from 

 the egg, are so deficient in the power of generating heat, that their tempera- 

 ture rapidly falls when they are removed from the nest and placed in a cold 

 atmosphere ; it being shown by collateral experiments, that the loss of heat 

 was not to be attributed to the absence of feathers, nor to the extent of sur- 

 face exposed in comparison with the bulk of the body; and that nothing 

 but an absolute deficiency in the power of generating it, would account for 

 the fall of temperature. This is quite conformable to facts well ascertained 

 in regard to Mammalia. The foetus, during intrauterine life, has little 

 power of keeping up its own temperature ; and in many cases it is much de- 

 pendent on external warmth for some time after birth. The degree of this 

 dependence, however, differs greatly in the various species of Mammalia, as 

 among Birds ; being less, in proportion as the general development is ad- 

 vanced. Thus, young Guinea-pigs, which can run about and pick up food 

 for themselves almost as soon as they are born, are from the first indepen- 

 dent of parental warmth ; whilst, on the other hand, the young of Dogs, 

 Cats, Rabbits, etc., which are born blind, and which do not for a fortnight 

 or more acquire the same development with the preceding, rapidly lose 

 their heat when withdrawn from contact with the body of the mother. 



439. In the Human species, it is well known that external warmth is 

 necessary for the Infant, its body rapidly losing heat when exposed to the 

 chilling influence of a low temperature; but the fact is too often neglected 

 (under the erroneous ideas of " hardening" the constitution) during the 

 early years of childhood. It is to be carefully remembered, that the develop- 

 ment of Man is slower than that of any other animal, and that his calorify- 

 ing power is closely connected with his general bodily vigor; and though 



1 [Smithsonian Contributions.] 



2 On the Influence of Physical Agents on Life, part Hi, chap. i. 



