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 afilux 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 rnedicatrix 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 apt to retain it longer than it had done when exposed to cold without any preparation of the kind implied ; it will resist impressions of cold longer after exercise than it would after a state of perfect quiescence ; the nervous system has acquired new energy; the economy is in a condition to react with greater effect than when depending on the process just described, that, namely, which takes place independently of the agency of the will. The repetition of the effects that follow exercise taken at due in tervals, hardens the frame to such a degree that the body at length acquires the power, by means of the insensible and involuntary reac tion alone, to resist degrees of cold which it could not have borne without the violent and voluntarily induced reaction of active muscular exertion.