IV. The nervous system presents diversities still more important and well-marked. The most striking character exists in the proportion of the principal trunk of this system, and especially of its encephalic extremity, which is much larger in the warm than in the cold blooded animals.
The most remarkable structural conditions of warm-blooded animals, then, are four in number, three of which are referable to the organs of nutrition, the fourth to the nervous system, which may be briefly related in the following order :-1. higher complication and greater extent of the digestive apparatus ; 2. entire separation of the circulating apparatus into two systems, the venous and arterial, with out direct communication between them ; 3. organs of aerial respiration presenting a much larger surface to the contact of the atmosphe rical air ; 4. a nervous system of which the axis, and especially the encephalic extremity, bears a very high ratio to the whole.
These structural characters determine the following modifications of function. 1st, The complexness and greater extent of the di gestive apparatus in warm-blooded animals produces a more perfect elaboration of the matters which serve for the formation of blood. 2nd, The arrangement of the parts of the cir culating system maintains the arterial blood quite distinct from the venous, and in a state of complete purity. 3rd, The respiratory ap paratus, by the great extent of its surfaces in contact with the air, secures that its distinguish ing qualities be imparted in the highest pos sible degree to the arterial blood, which more over is elaborated in larger quantity. The predominance of their nervous system, and es pecially of its encephalic extremity, renders all the parts of the body much more excitable, and gives the greatest energy to the nutritive functions. The whole of these organic condi tions are mutually dependent, and may be reduced to the expression of these two general conditions :—lst, the formation and distri bution of the arterial blood, the particularly exciting and nutritive blood of the body ; 2nd, the most powerful influence of the nervous system.
As these characters of primary significance in the animal economy coincide in Mammalia and Birds with the greater production of heat, and thus distinguish them from all other ani mals, it is probable that between these organic conditions and caloricity or the power of evolving caloric, there is a relation of the na ture of cause and effect. It is even almost impossible that this should be otherwise than as it has been stated ; for the characters of organization and the peculiarities of function, coincident with the greater evolution of calorie, are almost the sole points of any importance that distinguish warm from cold-blooded ani mals.
It is therefore nearly certain that the condi tions requisite to the production of heat must exist within the circle of the functions which we have described. And if this relation do actually exist—as these functions are in a state of mutual dependence,—it follows that one of them cannot be modified, the others remaining, so to speak, in the same condition, without modification resulting in the calorific capacity likewise. It is of great consequence
to verify this assumption, because if it be well-founded, the probability already elicited of the power of engendering heat being de pendent on the state of the functions in the relations which have been indicated, becomes matter of certainty. So that it is of the highest import to follow the modifications of these functions presented by animals and man in order to compare them with the respective varieties of calorific power presented by each. And if we find that they coincide, and accord with the principle established, we shall have discovered the conditions of organization and of function upon which the production of ca loric depends.
Conditions of organization and of func tions may be entitled the physiological causes of the production of animal heat. If we succeed in determining these, we ought to rest satisfied. If, indeed, to this knowledge we could add that of the immediate cause of this phenomenon among animals, or what is the physical cause, it would be a great gain for science. This, accordingly, was the ob ject of the labours of the majority of phy siologists who have given their attention to the subject of animal heat. But they could not possi bly succeed in their researches, for the simple reason that natural philosophers themselves have not yet discovered how heat is produced in the inorganic world ; although indeed they have presumed that they were acquainted with it. It is not to be wondered at, then, that attempts have been made to detect this presumed cause amidst the complicated phenomena of life. But natural philosophers have lost confidence in the theory which they had formed, and are searching for a new one. Meantime they are doing what ought always to be done under such circumstances ; they are studying with care the various conditions and circumstances in which it is produced ; determining these with precision, and measuring with rigour the quantity of heat produced. Of late, therefore, many distinguished physiologists have entered on the same path, and by experiment have endeavoured to ascertain the physiological con ditions of the production of heat. But if their predecessors have not attained the object they had in view, they have nevertheless ren dered very essential services to science ; for in searching after the physical cause of heat, they have determined with precision the physiological conditions of the production of animal heat, which are of very great importance. Inde pendently of the simple observation of the actual temperature of animals, the labours of physio logists on this subject consist almost entirely of experimental facts, that is to say, facts created by science.