106 



contractile, from the circumstance of their being stimulated respec- 

 tively by arterial and by venous blood. He considers the power of 

 bearing suspended respiration as a. measure of irritability, which may 

 be expressed by the length of time during which an animal can sup- 

 port the suspension of this function. He then shows that, conform- 

 ably to these definitions, the foetus before birth, the reptile, and the 

 molluscous animal possess a much higher degree of irritability than 

 the adult, or than animals belonging to the class of mammalia and 

 birds; in which the quantity of respiration being very great, the 

 irritability is proportionally small. 



He then proceeds to consider the phenomena of hybernation ; 

 and shows that they are very similar to those of the ordinary sleep 

 of the same animals, but differ from those of the sleep of animals 

 which do not hybernate. In the former case the respiration is 

 nearly, if not wholly, suspended, and the temperature greatly re- 

 duced ; but the circulation continues unimpaired. He notices dif- 

 ferences also in the habits of different hybernating animals, some of 

 which frequently awake from their slumber during the winter, while 

 with others the lethargy is uninterrupted. The state of hybernation 

 should, he thinks, be carefully distinguished from the torpor induced 

 by excessive cold ; the former being a conservative, the latter a 

 destructive process. The exclusion of atmospheric air, which is 

 speedily fatal to the animal in its active state, is sustained with per- 

 fect impunity during hybernation, the respiration being then entirely 

 suspended. The animal being at such times reduced to a state 

 analogous to that of the reptile, but in a still higher degree, the 

 irritability is much increased: the heart continues to beat without 

 the stimulus of aerated blood, and the circulation is kept up with 

 perfect regularity. This latter fact was ascertained by actual ob- 

 servation in the case of the bat, by adjusting the wing of the animal, 

 so as to admit of its being placed in the field of a microscope with- 

 out disturbing its repose. The experiments of Mangili are quoted 

 in proof of the longer continuance of the action of the heart after 

 decapitation, if the experiment be made in the hybernating state, 

 than if it be made when the animal is in its ordinary state of acti- 

 vity. 



Animals, during hybernation, are easily roused from their le- 

 thargy, and restored to sensibility and activity ; and the muscles do 

 not appear to be affected with the slightest rigidity : the respiration 

 is immediately resumed, and the temperature rises rapidly to the 

 natural standard. The hedgehog and the dormouse awake periodi- 

 cally from the sense of hunger, and the food then taken conduces 

 to renewed lethargy. But frequent excitation from this state is 

 productive of great exhaustion, and is often fatal to the animal. 

 Severe cold, like other causes producing a painful impression, rouses 

 the hybernating animal from its state of lethargy ; and if continued, 

 induces a state of torpor, which ends in death. 



