CHAP, v.] NUTRITION. 859 



adapted to undergo the change at the appointed season, and a 

 dormouse may fall into winter sleep at a temperature in winter 

 higher than that at which it awakes in spring. 



The phenomena of the hibernating mammal may be described 

 as those due to a lower rate of metabolism and hence to lowered 

 activity of the tissues in general. The heart beats very slowly, 

 each beat being at best of but moderate strength ; and the breaths 

 are few, feeble and far between. Respiration and circulation are 

 thus going on, but go on so to speak at almost the slowest possible 

 rate consistent with the continuance of the working of the economy. 

 The breaths are, as we have said, few and far between, but they 

 suffice to carry to the tissues the small amount of oxygen which 

 these need and to carry off the small amount of carbonic acid 

 which they produce. So small is the respiration of the tissues that 

 in the depths of the winter sleep the venous blood is almost as 

 bright as the arterial, the colour of which is nearly normal. And 

 the small amount of destructive katabolic changes which is going 

 on is shewn by a change in the respiratory quotient ; oxygen is 

 taken up out of proportion to the carbonic acid expired. Indeed, 

 it has been observed that a dormouse actually gained in weight 

 during a hibernating period; it discharged during this period 

 neither urine nor faeces, and the gain in weight was the excess of 

 oxygen taken in over the carbonic acid given out. 



As far as regards the other functions of the body all that can at 

 present be said is that the several fundamental activities of the 

 various tissues, though lowered, are still continued very much as 

 usual. The muscles and nervous elements are irritable; indeed 

 the hibernating animal may be awakened, though with difficulty, 

 by adequate stimulation ; and as an instance of the fundamental 

 similarity of the sleeping with the waking condition we may say 

 that the slowly beating heart can during hibernation be still 

 further slowed or be arrested by stimulation of the vagus nerve. 

 The essential feature of hibernation in fact is that external cold is 

 not resisted by the thermotaxic nervous mechanism, but lowers the 

 metabolism of all the tissues, and thus lowers the functions of the 

 whole body. When even in deep winter the hibernating animal 

 is exposed to adequate warmth, the increased temperature awakes 

 the tissues to increased metabolism, and the awakened animal 

 regains the bodily temperature and acquires all the powers which 

 it possessed in midsummer. 



Preparatory to the oncoming of hibernation the body lays up 

 unusually large stores of fat for the winter's expenditure. Many 

 hibernating animals possess a 'hibernating gland,' the cells of 

 which become loaded with fat in the autumn and lose it during 

 hibernation; but in all cases the great store of fat is in the 

 adipose tissue generally. The liver of the hibernating animal, at 

 all events of the* dormouse, contains a considerable quantity of 

 glycogen, which may be regarded as quite comparable to the 



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