ANIMAL HEAT. 



653 



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. We 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. 



We shall first compare Mammalia and Birds 

 to determine which of the two classes, in con- 

 formity vvith 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 

 38, 4 (101 F.), that of Birds 42 1 (108 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. 



II. The blood of Reptiles is superior to that 

 of Fishes both as regards the nature of the 

 globules and their relative proportion, their size 

 being smaller, and their numbers greater, than 

 among Fishes. 



If the whole of the blood in the Reptile is 

 not transmitted through the organ of respira- 

 tion, whilst in the Fish it is, a larger quantity 

 of this fluid is brought into contact with the air 

 in the same space of time in consequence of 

 the greater extent of surface of the organ in the 

 Reptile, and then the Reptile has the farther 

 immense advantage of a pulmonary or aerial 

 respiration, whilst that of the Fish is branchial 

 or aquatic. To conclude, the nervous system 

 of Reptiles is much more developed in the 

 cerebro-spinal axis, and especially in the ence- 

 phalic extremity, than in Fishes. 



From this comparison it follows that the 

 organic and functional conditions, judging of 

 these in conformity with the principles which 

 we have taken as our guide, are much more 

 favourable to the development of heat in Rep- 

 tiles than in Fishes. This theoretical deduction 

 is fully confirmed by direct observation, as we 

 have seen above, and this verification becomes 

 a new confirmation of the accuracy of the prin- 

 ciple. 



We continue to pursue this parallel by a 

 summary comparison of the organization in its 

 relations with the production of heat in the 

 cold-blooded Vertebrata and the Invertebrata 

 generally. A glance suffices to shew the vast 

 inferiority of the Invertebrata in this as in every 

 other respect. In the first place their blood is 

 so little of the same nature as that which has 

 been recognized most favourable to the produc- 

 tion of heat, that it wants the characters whe- 

 ther of arterial or of venous blood. The blood 

 of the Invertebrata, with the exception of a 

 very small group (the worms with red blood), 

 is colourless. In the structure and number of 

 its globules it is also greatly inferior. The 

 globules, indeed, may be smaller, but then they 

 are of a much more simple structure, and con- 

 sequently lower in the scale, in other words 

 more imperfect. In the relation to the fluid 

 part of the blood too, they are in much smaller 

 proportion than among the Vertebrata. An 

 analogous character is manifest in the tissues 

 generally, the proportion of water in them 

 being incomparably larger in the Inverte- 

 brate than in the Vertebrate series of animals. 

 Finally, there is an immeasurable inferiority in 

 the nervous systems of the Invertebrate com- 

 pared with even the lowest of the Vertebrate 

 series of animals. 



Fiom all this it results, agreeably to the 

 principle of which we are now showing the 

 application, that the Invertebrate ought to have 

 a much smaller capacity of producing caloric 

 than even the cold-blooded Vertebrate animals; 

 and this is exactly as we found matters to be 

 by direct experiment in regard to the tempera- 



