652 



ANIMAL HEAT. 



the greatest possible number of phenomena 

 connected with animal heat ; and in determin- 

 ing the physiological conditions of its produc- 

 tion, we shall lay up a store of theoretical 

 knowledge peculiarly applicable to practice, 

 the end and object of all physiological inves- 

 tigation. 



The means of comparing these modifications, 

 however, and of judging of their importance 

 are not always easy. We shall do as much as 

 the actual state of our knowledge permits if 

 we inquire first, by what means we can ap- 

 preciate the modifications relative to the arterial 

 blood. 



1. As regards the quantity of the arterial 

 blood, we shall view this point of the inquiry 

 less with reference to the whole amount of 

 blood circulating in the body, than to the 

 quantity which is formed at a time, as it were, 

 in the lungs; because it is evident that if the 

 arterial blood influences the phenomenon of 

 heat, the more that is formed at any given 

 time the greater ought to be the direct or in- 

 direct influence upon the production of heat. 

 . As it is not always possible to have a direct 

 and precise measure of the relative quantity of 

 blood in the organs, we must be content with 

 an approximative mode of estimating this, 

 which consists in ascertaining in what degree 

 the lungs are loaded with blood, b. An aid to the 

 judgment may also be derived from the relative 

 size of the lungs, the tissue being presumed to 

 be nearly alike throughout their entire mass. 

 c. With an equal volume of lungs, the greater 

 or less compactness of the tissue must be taken 

 into the account. The closer the tissue is, 

 the more are the surfaces in contact with the 

 air multiplied, d. The extent and rapidity of 

 the respiratory motions form another element 

 in the calculation ; for to increase the amount 

 of relation with the air is analogous to the for- 

 mation of a larger quantity of arterial blood 

 within a given time. 



All the foregoing data refer to the absolute 

 or relative quantity of arterial blood. But 

 there are other particulars connected with its 

 constitution which it is necessary to mention. 

 The blood, for instance, is composed of a fluid 

 and solid part, the latter existing under the 

 form of globules. It is obvious that the fluid 

 is not the characteristic part of the blood, in- 

 asmuch as this is met with elsewhere, whilst 

 the globules of the blood are only known as 

 constituents of this fluid. The arterial blood 

 consequently ought to have qualities by so much 

 the more distinctive and energetic as it con- 

 tains a larger proportion of globules. Now 

 this is a character that may be appreciated with 

 exactness, and measures of it have been given. 

 But the globules of the blood are not in- 

 variably of the same nature, a fact which may 

 be judged of by outward and very obvious and 

 appreciable characters, namely, size and form. 

 The smallness and more or less perfectly sphe- 

 rical or rounded form of the blood-globules 

 distinguishing animals with warm blood, co- 

 incide in the Vertebrata with a higher capacity 

 to produce heat. For we do not institute this 

 comparison here save in reference to animals 



included in this division, inasmuch as the cha- 

 racters of the blood have only been studied 

 under these relations among them. We shall, 

 therefore, hold the energy of the calorific power 

 to be connected with the smallness and rounded 

 form of the globules of the blood in vertebrate 

 animals. 



2. The materials of the blood being sup- 

 plied by the digestive apparatus, we might 

 judge, all things else being equal, of the per- 

 fection of the blood by the perfection of this 

 apparatus. But there is likewise a necessary 

 co-relation between the result of the function, 

 and the aliment; for instance, when the ap- 

 paratus shall be found nearly alike in any two 

 cases, the difference of food necessarily in- 

 fluencing the qualities of the blood, the com- 

 parison must be established, every other cir- 

 cumstance being equal, according to the higher 

 or lower nutritive qualities of the food. 



As the use of the arterial blood is to excite 

 and nourish the different parts of the body, 

 there will be a necessary correspondence be- 

 tween the blood and the result of the nutrition 

 which may become manifest in the nature and 

 quality of the tissues. And in this case it 

 would be fair to make use of these characters 

 of tissues to form an estimate of the nature of 

 the blood in reference to its aptitude to pro- 

 duce heat ; and this we shall accordingly do. 

 But even in the event of all these characters 

 failing us, there is another source whence we 

 can derive comparative measurements, which 

 are susceptible of very rigorous application. 



Since it is necessary that the venous blood 

 should pass through the lungs in order to be- 

 come arterial from contact with the air of the 

 atmosphere, it is obvious that it cannot un- 

 dergo any change in its constitution without 

 the air at the same time suffering a change. 

 That the air is altered by the respiratory act is 

 well known to all, and as there is a necessary 

 co-relation between the blood aerated during 

 respiration and the air which it alters, the 

 amount of alteration undergone by the one 

 may be estimated from the change suffered by 

 the other. The quantity of air altered by re- 

 spiration, all other things being equal, ought 

 to be found in relation with the production of 

 heat. 



The different characters which we have men- 

 tioned all refer directly or indirectly to the 

 blood. There still remains one of another 

 order which may also serve us as a guide in 

 making comparisons in reference to the pro- 

 duction of heat. The allusion here made is 

 to the nervous system, the superior value of 

 which in warm-blooded animals has already 

 been commented on. It is thus, then, that we 

 may assume the predominance of the nervous 

 axis, and particularly of its encephalic ex- 

 tremity, as a condition favourable to the pro- 

 duction of heat, and which, in circumstances 

 of parity among the other conditions, must 

 tend to the production of a greater quantity of 

 heat. Such are the modes of proceeding which 

 we shall follow in investigating the modi- 

 fications of the organic conditions and of the 

 functions which coincide with the greater evo- 



