ANIMAL HEAT. 



673 



tion which characterizes them, the fundamental 

 principle or distinguishing feature of which is 

 a defect of energy, or of power of reaction. 

 This principle", so simple in itself, artd which 

 is but the true expression of the various facts 

 reduced to unity, renders plain and obvious 

 much that otherwise appears anomalous or 

 contradictory. In studying the phenomena of 

 animal heat under new relations, we shall find 

 the confirmation of what precedes. 



OF THE SYSTEM UPON WHICH THE EXTERNAL 

 TEMPERATURE ACTS PRIMARILY AND PRIN- 

 CIPALLY. 



Our sensations admonish us that it is the 

 nervous system that is acted upon primarily and 

 principally by changes of external temperature. 

 In the first place the impression is felt instan- 

 taneously ; in the second place the intensity of 

 the sensation is in relation with the degree of 

 external heat or cold ; in the third place the im- 

 pression is not limited to the various degrees of 

 the corresponding sensation of heat or cold ; it 

 extends to the other faculties of the nervous 

 system, increasing or diminishing the general 

 or special sensibility ; in the fourth place it acts 

 powerfully in increasing or diminishing the 

 activity of the muscular system, principally 

 through the medium of the nervous system. 



Influence of temperature on the vitality of 



cold-blooded animals. 



If the functions of respiration and general 

 circulation be destroyed by the excision of the 

 lungs and heart of a cold-blooded animal, of 

 one of the Batrachia for example, life may still 

 continue for a time. Of the three principal 

 systems of the economy the only one then left 

 untouched is the nervous ; so that the animal 

 may be viewed as living almost exclusively by 

 the agency of this system. If several animals in 

 this condition be plunged in water deprived of 

 air, they will live in it different spaces of time 

 according to the degree of its temperature, 

 the extremes compatible with their existence 

 being zero and 40 c. It is towards the 

 inferior limit, zero, that they live the longest. 

 Towards the upper limit they die almost im- 

 mediately. Temperature, consequently, pre- 

 sents in the scale of variations just mentioned 

 very remarkable relations with the vitality of 

 these animals. Towards the lower limit or that 

 of melting ice, it is obviously most favourable to 

 life; towards the upper limit, it is most inimical 

 to life, extinguishing it almost immediately. 

 Here it is impossible to mistake the system 

 upon which the variety of temperature exerts 

 its first and principal effects the nervous 

 system. 



If respiration only be annihilated by 

 plunging these creatures under water deprived 

 of air, the temperature of which is caused to 

 vary as above, they will be found to present the 

 same phenomena according to the degree of 

 the heat or cold, but in a more striking 

 manner. Temperature in this case has the 

 same kind of influence, but the effects are more 

 manifest, from the circulation of the venous 

 blood prolonging life at every degree short of 



VOL. II. 



the one at the upper limit of the scate, at which 

 life is extinguished quite as suddenly as in the 

 former instance. 



Such are the direct and instantaneous effects 

 of temperature upon the vitality of cold- 

 blooded animals. But there are others which 

 flow from its successive agency, during a con- 

 siderable length of time. If the series of ex- 

 periments just quoted be made in summer, 

 and the different lengths of life at different 

 degrees of temperature of the frogs immersed 

 in water be noted, (between the limits which we 

 have pointed out above,) and the same expe- 

 riments be repeated in autumn, the length or 

 tenacity of life manifested by the animals will 

 be much greater at the same degrees of tem- 

 perature they will in general be found to live 

 twice as long now as they did in summer, at 

 corresponding and equal temperatures of the 

 medium in which they are immersed. The 

 depression of atmospheric temperature in the 

 autumn has modified their constitution, and 

 actually increased their vitality, precisely in 

 the manner indicated above. The slight 

 effects of each successive fall in the general 

 temperature have accumulated in the constitu- 

 tion so as to render their vitality or tenacity of 

 life much greater, a fact which is made abun- 

 dantly manifest by the faculty of the animals 

 to remain for a much longer time immersed in 

 water without breathing than they could have 

 done in summer. If a third series of experi- 

 ments of the same description be made in 

 winter, the tenacity of life will be found to 

 have increased in a very high degree. At the 

 same degree of temperature frogs will be found 

 to live immersed in water deprived of its air 

 at least twice as long in winter as they could 

 have done in autumn. The same cause the 

 depression of the atmospheric temperature 

 has continued to act with greater intensity 

 and for a longer period, and the constitution, 

 gradually modified by greater and longer con- 

 tinued cold, has acquired greater tenacity of 

 life. 



The opposite effect takes place with the 

 successive rises of the temperature from that of 

 winter to that of summer ; so that among cold- 

 blooded animals the maximum of vitality, 

 i. e. tenacity of life, corresponds to the depth 

 of winter, the minimum to the height of sum- 

 mer. The slight and from moment to moment 

 inappreciable effects produced by the external 

 temperature, whether tending to increase or to 

 diminish the vitality, accumulate with their 

 repetition through the period of each season, 

 and produce a corresponding change in the con- 

 stitution with regard to tenacity of life. These 

 accumulated effects of the different portions of 

 the year constitute the influence of the seasons 

 on the constitution with respect to many of the 

 most important relations of life. The first of 

 these we have just examined cursorily that is, 

 the faculty of living in air according to the 

 influence of the actual temperature, or of that 

 of the past temperature, in other words the 

 season that has immediately preceded. The 

 second of these fundamental relations consists 

 in the various proportions of air necessary to 



2 Y 



