INFLUENCE OF HEAT ON ANIMALS. 83 



cold water, is frequently in the highest degree beneficial, by imparting 

 tone to the system, i. 0., by producing a firmer condition in the solids 

 which were previously relaxed^ and more especially by calling into action 

 the tonicity of the walls of the blood-vessels, which imparts to them an 

 increased resistance, and thus favours the regular and vigorous circu- 

 lation of blood, upon principles which will be hereafter stated ( 609). 

 But so far from producing any permanent depression in the temperature 

 of the body, this measure has a tendency to elevate it, by the increased 

 vigour it produces in the circulation ; hence the glow which is experi- 

 enced after the use of the cold bath. If this effect be not produced, and 

 a chilling of the body, instead of an invigorating warmth, be the result 

 of the use of cold, it is evident that this cannot be beneficial. The inju- 

 rious results of the too-prolonged application of even a moderate degree 

 of cold, are seen in the depression of temperature, without a correspond- 

 ing reaction, which is the consequence of an immersion in water of 50 

 or 55 prolonged for several hours ; and still more in that chilling of the 

 whole surface, frequently productive of the most serious consequences, 

 which arises from the evaporation of fluid from garments that have been 

 moistened, either by perspiration from within, or by the fall of rain or 

 dew upon their exterior. There is no doubt that the obstruction to the 

 continuance of the perspiration, presented by a covering already satu- 

 rated with moisture, is one cause of the injurious results that so com- 

 monly follow such an occurrence ; but there is as little doubt that the 

 chilling influence of the external evaporation has a large share in pro- 

 ducing them. For experience shows that, if the evaporation be pre- 

 vented by an impenetrable covering, the contact of a garment thoroughly 

 saturated with moisture is not productive of the same injurious conse- 

 quences. 



120. The practical importance of the due comprehension of the prin- 

 ciples, upon which Heat and Cold should be employed, in the treatment 

 of disease and the preservation of health, has required this digression. 

 We now proceed to consider the influence of temperature upon a certain 

 group of warm-blooded animals ; which offers a remarkable peculiarity 

 in this respect, their power of generating heat being for a time greatly 

 diminished or almost completely suspended ; the temperature of their 

 bodies following that of the air around, so that it may be brought down 

 nearly to the freezing-point ; their general vital actions being carried 

 on with such feebleness as to be scarcely perceptible ; and yet the vital 

 properties of the tissues being retained, so that, when the temperature 

 of the body is again raised, the usual activity returns. This state, which 

 is called hibernation^ appears to be as natural to certain animals, as 

 sleep is to all, and it corresponds with sleep in its tendency to periodical 

 return. 



121. No account can be given of the causes to which it is due ; but 

 the condition of the animals presenting it offers several points of much 

 interest. There are some, as the Lagomys, in which it appears to differ 

 but little from deep ordinary sleep ; they retire into situations which 

 favour the retention of their warmth ; and they occasionally wake up, 

 and apply themselves to some of the store of food, which they have pro- 

 vided in the autumn. In other cases, a great accumulation of fat takes 



