580 GENERAL REVIEW OF THE NUTRITIVE FUNCTIONS. 



cation, also, is derivable from the knowlecjge which we thus gain, of the im- 

 portance of the Cutaneous Respiration ; for it leads us to perceive the desira- 

 bleness of keeping the skin moist, in those febrile diseases in which there are 

 great heat and dryness of the surface, since secretion cannot properly take 

 place through a dry membrane. Of the relief afforded by cold or tepid spong- 

 ing in such cases, experience has given ample evidence. 



727. It has been, and still is, a prevalent opinion amongst Physiologists of 

 the Chemical school, that the process of Calorification is one of ordinary Com- 

 bustion ; being entirely dependent upon the union of Carbon and Hydrogen, 

 either directly furnished by the non-azotized constituents of the food, or 

 derived from the disintegration of the tissues ( 433), with the Oxygen 

 obtained from the atmosphere. It may be questioned, however, on several 

 grounds, whether this is a sufficient account of it. From the experiments of 

 Dulong and Despretz (between which there is a close agreement), it appears 

 that more Heat is given off from the bodies of warm-blooded animals than 

 can be accounted for by the union of the amount of Carbon and Hydrogen, 

 contained in the Carbonic acid and Watery vapour exhaled by them during a 

 given time, with their equivalents of Oxygen. According to Dulong, the 

 combustion of the Carbon alone would not account for more than half of the 

 caloric liberated by Carnivorous animals, or for more than seven-tenths of that 

 set free by Herbivorous species ; and, even when the Hydrogen was also 

 taken into account, the amount of heat accounted for was only from 3-4ths to 

 4-5ths of that which is developed in the same space of time. The results 

 obtained by Despretz were very similar ; for he found that the Heat that 

 would be generated by the union with Oxygen of a given amount of Carbon 

 and Hydrogen, was only from 3-4ths to 9-10ths of that which would be set 

 free by an animal, in the time necessary to exhale from the skin and lungs a 

 corresponding amount of Carbonic acid and of Watery vapour. Although the 

 attempt has been made by Liebig to invalidate these results, yet there are 

 other reasons that tend to the same conclusions. Thus we have no proof 

 whatever that the Watery vapour which passes off from the Lungs, is, any 

 more than that transpired from the skin, a product of the Combustion of Hy- 

 drogen ; it is just as likely to be a simple product of exhalation from the 

 Blood, recently diluted by the introduction of Chyle, which contains a large 



proportion of fluid. Again, there are many other uses for the Oxygen ab- 

 ~sorbed from the air (according to Liebig's own showing) within the system ; 

 such, for example, as the oxidation of the Sulphur and Phosphorus, which 

 pass off in the form of acids by the Urine ; or the combination of Oxygen, in 

 various proportions, with Protein, in its metamorphosis into the elements of 

 the various tissues. Hence it is evident that the Chemical doctrine, in its 

 present form, is insufficient to explain the phenomena of Animal Calorifica- 

 tion ; but there can be little doubt that an increased knowledge of the Molecu- 

 lar changes which go on within the system, will afford a solution of the diffi- 

 culty. At present, then, it may be stated as a general fact, that the production 

 of Animal Heat is due to the various changes in Chemical composition that 

 are continually taking place within the system ; of which changes, the absorp- 

 tion of Oxygen, and the disengagement of Carbonic Acid, are the two chief 

 external manifestations : and that the degree of Caloric liberated bears a 

 close relation to the activity of these changes, either in regard to the body at 

 large, or to any portion of it. 



728. The researches of Dr. Edwards upon Animal Heat have brought to 

 light some very interesting facts regarding the diversity which exists as to the 

 power of generating heat in the same species of animal at different ages, 

 and at different periods of the year. It appears to be a general fact that the 

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



