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 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 he 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 lution of heat. To ascertain 'whether this coincidence is to be viewed as being in the mutual relation of cause and effect, it imports to know whether or not their variations are in relation with those of the heat produced. If they coincide whenever we compare them, pro vided these comparisons are but sufficiently numerous, we shall be safe in admitting a necessary connexion between them. 1Ve were led to the relation which engages our attention in the course of our comparisons of warm blooded animals with those having cold blood considered in general. Let us now enter upon a comparison of the same kind, but more par ticular, whilst we take account of the most important subdivisions of these two great groups in order to verify our first inductions.
e shall first compare Mammalia and Birds to determine which of the two classes, in con formity with the principle to which we have been led, has its organization most favourable to the production of heat.
The lungs of Birds, although smaller, are more loaded with blood than those of the Mammalia, and are in communication with ex tensive air-cells, spreading all through the body and even penetrating into the cavities of the bones, so that the air may be said to penetrate the body generally, and to be in contact with the ramifications of the aorta as well as with those of the pulmonary artery ; the blood of these animals is therefore in the most extensive rela tion imaginable with the air of the atmosphere. Again, if the nature of the blood of Birds be considered, independently of this extensive relation with the air, the organic condition here will not appear less favourable to them. The globules of this fluid, indeed, are a little larger and less spherical than in Mammalia, which is a disadvantage; but the proportion they bear to the fluid part is so favourable to Birds that this circumstance must give them immensely the advantage in reference to the character which engages us. With regard to the nervous sys tem, if the encephalic extremity is developed in a minor degree in Birds, their circulating and respiratory systems act with greater quick ness. Lastly, and as an effect of the whole of these conditions,the consumption of air is much greater among Birds than among Mammalia.
From all that precedes, it follows, if the principles already laid down be correct, that Birds ought to produce the greatest quantity of heat; and this is actually the case, as we have seen when we were speaking of the actual tem peratures of the different classes of animals— the mean temperature of the Mammalia is 4 F.), that of Birds 42° t (103 F.). Here, then, is a powerful confirmation of the relation which we have recognized between the conditions of the organization and the produc tion of heat; it is of so much the more value as the relation being based on the comparison of two classes so numerous, the verification is made on a scale of proportionate extent. We shall extend it still farther by contrasting in the same manner the two other classes of the Ver tebrata, Reptiles and Fishes.
I. The organs which prepare the materials of the blood—the digestive apparatus is more complete among Reptiles than among Fishes ; 1st, in the dental apparatus when it exists ; 2d, in the more distinct stomach ; 3d, in the greater length of the intestines.