ANIMAL HEAT. 



67-5 



The analogy on either hand consists in the 

 effects of temperature. But the differences 

 that must necessarily occur between natures 

 that vary in so many other respects are espe- 

 cially encountered in the dissimilar effects of 

 cold. Here we observe a general compensa- 

 tion which distinguishes in the most marked 

 manner the Vertebrata having a constant or all 

 but a constant temperature, from the hyber- 

 nating tribes or Vertebrata whose temperature 

 varies, and the cold-blooded series generally. 

 The relation of cold, or of a low temperature 

 relatively to the standard of the more perfect 

 beings of creation, is one of essential impor- 

 tance, and requiring our most careful investi- 

 gation. 



Cold, as has been said, tends to diminish 

 sensibility and motility; but cold itself is per- 

 ceived by causing a diminution of the general 

 sensibility ; among animals of superior organi- 

 zation it even acts indirectly as a stimulus : 

 the blood flows into the parts that had" been 

 chilled, if their temperature has not fallen 

 too low, for then all sensibility is extinguished 

 and reaction never occurs. The afflux of 

 blood to the external parts is manifested by 

 the increased redness ; and the skin becomes 

 red in proportion as the parts it covers are 

 susceptible of acquiring a high temperature, 

 such as the hand. We have shown that the 

 consequence of the afflux of blood is an in- 

 crease of temperature which tends to counter- 

 balance the effects of the refrigeration. The 

 compensation, however, is not perfect. For 

 in winter the temperature continues above that 

 of summer, although there is a greater pro- 

 duction of heat in winter than there is in 

 summer, as we have shown above. 



The constitution of the Vertebrata having a 

 nearly constant temperature differs essentially 

 in the power of reaction it possesses ; a power 

 which cannot better be expressed than by the 

 word energy, and which must necessarily be 

 referred to the nervous system. The power of 

 reaction under the influence of cold is exhi- 

 bited in two modes : the first is that which has 

 just been mentioned, in which the stimulus of 

 the cold calls the blood into the capillaries of 

 the surface, without exciting any kind of vio- 

 lent motions in the circulating and respiratory 

 systems ; the second consists essentially in 

 this last kind of excitement. The sharpness of 

 the cold stimulates the respiratory motions, 

 which become accelerated, and the quickening 

 of the motions of the heart follows or accom- 

 panies those of the lungs. These two modes 

 of reaction must be viewed as two degrees of 

 the same power : 1st, an afflux of the blood to 

 the capillary vessels ; 2d, acceleration of the 

 motions of the thorax and heart. There is, 

 however, between these two processes a diffe- 

 rence which it is of the greatest conse- 

 quence clearly to understand. The first, so 

 long as it remains within certain and suitable 

 limits, is a reaction that maintains the eco- 

 nomy in a state of health. The second tends 

 to produce salutary effects, but becoming ex- 

 cessive it brings the body into a state of disease. 

 The first is sufficient to enable those creatures 



whose system is energetic to resist the effects 

 of rigorous cold, by preserving their general 

 activity and the normal state of their functions. 

 The second is the resource of those animals, 

 which, although of the same species, are so 

 constituted that the energy of the nervous sys- 

 tem is less than in the former. This is what 

 occurs universally in very early life. It is a 

 reaction the tendency of which is salutary, but 

 which is not the less on this account the 

 index and essence of a proper pathological 

 state. It is one of the cases in which the vis 

 medicatrix nature is peculiarly and most 

 strikingly manifested. This position is made 

 singularly evident by the following experi- 

 ment : when a young bird, bare, or but scan- 

 tily covered with feathers, is taken from the 

 nest, and exposed to the open air, even in the 

 summer season, its respiration will be seen to 

 be accelerated in the ratio of the cold it expe- 

 riences. It is peculiarly worthy of remark 

 that this salutary reaction, taking place under 

 the influence of the nervous system, acting, in 

 the case quoted, independently of the will, is 

 in a great measure the same as that which we 

 bring into play by means of the will to com- 

 bat the same evil. When in health, for instance, 

 we are exposed to and feel the impression of 

 cold severely, and have no resource but in our- 

 selves, we begin immediately to take exercise, 

 and move about; and if we do this with sufficient 

 vigour, the motions of respiration and circula- 

 tion are very soon increased in rate, and 

 our heat returns ; it being always understood 

 that the external cold is not at too rigorous 

 a degree. From what precedes, we are in a 

 state to appreciate the part which each func- 

 tion has in causing the developement of heat by 

 exercise. The experiments of Messrs. Bec- 

 querel and Breschet, referred to in an early 

 part of this paper, have proved that the con- 

 traction of the voluntary muscles is accom- 

 panied by the evolution of caloric, and that 

 the heat increases by a succession of muscular 

 contractions. The first source of the heat 

 evolved in exercise, therefore, lies in the con- 

 tractions of the muscles, that is, in the volun- 

 tary motions. These, vigorously called into 

 play, are followed by increased rapidity in the 

 action of the muscles of respiration, and of 

 the central muscle of circulation, the heart; 

 and these, by the increased energy they impart 

 to the functions over which they preside, cause 

 an increase in the temperature in conformity 

 with the general principles already laid down. 

 It is well to follow the effects of exercise in the 

 various modifications under the influence of 

 cold. They produce phenomena which extend 

 farther than the state of health, and which ap- 

 pear in other conditions and circumstances 

 from analogous reasons. Exercise, according 

 to its degree and the degree of temperature 

 of the external air, is adequate not only to 

 compensate a chill, and to restore the body to 

 its pristine temperature in every part, but even 

 to do more than this. If the exercise has been 

 sufficiently prolonged, but not been excessive, 

 it may be suspended ; and the body, now re- 

 stored by its means to its temperature, will be 

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