650 



ANIMAL HEAT. 



no more than five species, but they belonged 

 to very different orders. The mean external 

 temperature being 22, 3 (72, I-.), that of 

 fishes was found to be but 23, 2 (74 F.), 

 which is very little more than one degree cen- 

 tigrade higher. This difference becomes even 

 more striking, if possible, as we descend in 

 the scale of animals. 



TEMPERATURE OF INSECTS. From eight 

 observations on Insects of very dissimilar spe- 

 cies, the mean temperature of the air being 

 24 (75, 5 F.), that of the insects was 24, 2 

 (75, 75 F.) 



TEMPERATURE OF THE CRUSTACEA. Two 

 species of Crustaceans, the cray-fish and crab, 

 presented a still more interesting phenomenon. 

 The mean temperature of the air was 24, 4 

 (76 F.) at the time of experimenting, that of the 

 Crustacea 24, 1, or somewhat lower than the 

 ambient medium. This we do not presume to 

 present as the rule, but we would say that the 

 temperature of the Crustacea is nearly equal 

 to that of the medium in which they are 

 plunged. 



TEMPERATURE OF THE MOLLUSCA. In 

 observing the temperature of a single Mollusc, 

 the common oyster, the temperature of the sea 

 being 27, 8, (82 F.) that of the animal was 

 27, 8 also. 



It is obvious, therefore, that the differences 

 in the temperature of animals from reptiles 

 inclusively downwards is very inconsiderable. 

 All these animals, indeed, may he united 

 under a single category, and regarded as con- 

 stituting a single group characterised by the 

 state or degree of their temperature. The 

 same may also be done with reference to the 

 animals of the two higher classes, Mammalia 

 and Birds, which in point of temperature are 

 so nearly akin to each other. 



There are consequently two grand divisions 

 of animals as regards temperature; the one 

 comprising the Mammalia and Birds; the 

 other including Reptiles, Fishes, Insects, Crus- 

 taceans, and Molluscs. The first is known 

 under the name of warm-blooded animals, the 

 second under that of cold-blooded animals. 



To characterize the first under the view of 

 temperature, the mean of the temperatures of 

 the respective classes which compose it must 

 first be taken. From the experiments of Dr. 

 Davy the mean temperature of Mammalia ap- 

 pears to be 38, 4 (101 F. ) 



that of birds 42, 1 (108 F. ) 



Mean of both classes 40, 25 (104, 5 F.) 



We may therefore say that the mean tempe- 

 rature of warm-blooded animals, including 

 man, surrounded by a moderate external tem- 

 perature is in round numbers 40 (104 F.) 

 between the limits of 36 and 44 (97 and 

 111, 5 F.), by which we have a scale of dif- 

 ference amounting to 8 (about 14 F.). 



The other class, that, namely, including the 

 cold-blooded animals, having no peculiar tem- 

 perature proper to them, may be characterized 

 in the following manner: their temperature 

 differs little or not at all from that of the sur- 



rounding media in which they live, when this is 

 at a degree which may be called moderate; so 

 that the differences are either inappreciable, or 

 do not exceed the limits of + 4 (39, 50 F.). 

 We shall return by-and-by upon this character, 

 which requires development. 



General conditions of organization in relation 

 with the production of' a greater or less 

 degree of heat. 



So wide a difference in the heat of the two 

 categories of animals might lead to the pre- 

 sumption that there is also a very great dif- 

 ference in point of structure. If, indeed, this 

 relation exists and is easily detected, we may 

 be led to discover the general conditions of 

 organization upon which the production of 

 heat depends. Is there an organization com- 

 mon to Mammalia and Birds, distinct and 

 different from that belonging to the other 

 classes of animals ? This question can be an- 

 swered in the affirmative : there is a well-marked 

 diversity of organization which distinguishes 

 Mammalia and Birds from all other animals. 



I. The most prominent feature of diversity 

 exists in the sanguiferous system, which is 

 divided through its entire extent into two dis- 

 tinct parts without direct communication be- 

 tween them, the heart presenting a complete 

 median septum, the bloodvessels in like man- 

 ner forming two systems of canals, which 

 have also no immediate communication in their 

 trunks. 



II. This peculiarity of structure, which is 

 only met with among animals having warm 

 blood, is regularly associated with an organ 

 adapted for aerial respiration. The character 

 which distinguishes this respiratory organ from 

 the one met with among cold-blooded animals, 

 reptiles especially, is this, that either in itself 

 or its appendices (the air-sacs of birds) it pre- 

 sents a much larger extent of surface in relation 

 with the air. 



III. Warm-blooded animals are farther dis- 

 tinguished from the cold-blooded by important 

 modifications of the digestive canal. 1. The 

 first portion of the apparatus from the mouth 

 to the stomach is much more complex in them; 

 for instance, it presents either a much more 

 perfectly developed dental system, fitted to 

 divide the food, or a sac, as among birds, 

 fitted to macerate the aliment, and cause it to 

 undergo a kind of preparatory digestion before 

 it is passed to the stomach. 2. The stomach 

 is more distinct; either the entrance to and 

 exit from this pouch are better marked, being 

 often provided with a valve, as in the Mam- 

 malia, or its structure and form are more spe- 

 cial, as we observe it among Birds. 3. The 

 intestinal canal is much longer in the warm 

 than in the cold-blooded tribes. 



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 



