EVOLUTION OF HEAT. 629 



generated through the immediate instrumentality of the Nervous system, not- 

 withstanding that the ultimate source of its development lies (as in the Chemi- 

 cal theory) in the oxidation of the elements of the food. Such an hypothesis 

 will be found consistent, the Author believes, with all the well-ascertained facts 

 of the case; for whilst it assigns their full value to all those proofs, which estab- 

 lish (in his mind) the necessary dependence of Calorification upon the changes 

 to which the Inspiration is subservient, and thus upon the supply of combus- 

 tive material on the one hand, and of oxygen on the other, it also assigns a defi- 

 nite modus operandi to the Nervous system, as an instrument largely concerned 

 in the production and distribution of the heat thus generated this modus ope- 

 randi, moreover, being in such complete harmony with the other manifestations 

 of Nervous power, that its existence might almost have been predicated upon 

 general considerations. 1 



664. We have now to inquire whether the power of generating Heat is pos- 

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

 very different from that of the usual temperature of the body at the various 

 periods of life; since an individual who can maintain a high temperature when 

 the surrounding air is moderately warm, may have very little $ower 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, &c. 3 It appears from these to be a general fact that, the 

 younger the animal, the less is its independent calorifying power. The de- 

 velopment of the embryo of oviparous animals is entirely dependent upon the 

 amount of external warmth supplied to it ; and 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 temperature rapidly falls when they are removed 

 from the nest and placed in a cold atmosphere. It was shown by collateral ex- 

 periments, that the loss of heat was not to be attributed to the absence of fea- 

 thers, nor to the extent of surface 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 intra-uterine life, 

 has little power of keeping up its own temperature ; and in many cases it is 

 much dependent 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 advanced. 

 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 independent of parental 

 warmth; whilst, on the other hand, the young of Dogs, Cats, Rabbits, &c., which 

 are born blind, and which do not, for a fortnight or more, acquire the same de- 

 velopment with the preceding, rapidly lose their heat when withdrawn from 

 contact with the body of the mother. 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 idea 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 calorifying 

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

 becomes more independent of it as development advances, it is many years before 

 the standard can be maintained without assistance, throughout the ordinary 

 vicissitudes of external temperature. Especial care is required with regard to 

 the maintenance of the bodily heat by artificial warmth, in the case of children 



1 See the Author's Memoir "On the Mutual Relations of the Vital and Physical Forces," 

 in "Phil. Trans.," 1850. 



2 "On the Influence of Physical Agents on Life," Part iii. Chap. i. 



