DR. MARSHALL HALL ON HYBERNATION. 
359 
7- There is an important distinction between true hybernation and torpor 
from cold, not attended to by physiologists. 
8. Severe cold, like all other causes of pain, rouses the hybernating animal 
from its lethargy ; and, if continued, induces the state of torpor. 
In conclusion, one of the most general effects of sleep, is to impair the respi¬ 
ration, and with that function, the evolution of animal temperature. The im¬ 
paired state of the respiration, induces a less arterial condition of the blood, 
which then becomes unfit for stimulating the heart; accumulation of the blood 
takes place in the pulmonary veins and left auricle; a sense of oppression is 
induced, and the animal is either roused to draw a deep sigh, or awakes alto¬ 
gether. 
Such are the phenomena in animals in which the heart has not the faculty 
of taking on an augmented state of irritability, with this lessened degree of sti¬ 
mulus. But in those animals which do possess this faculty, a property which 
constitutes the power of hybernation, the heart continues the circulation of the 
blood, more slowly indeed, but not less perfectly, although its arterial character 
be diminished and its stimulant property impaired. No repletion of the pul¬ 
monary veins and of the left auricle, no sense of oppression is induced, and the 
animal is not roused ; the respiration continues low, the temperature falls, and 
the animal can bear, for a short period, the abstraction of atmospheric air. 
All the phenomena of hybernation originate, then, in the susceptibility of 
augmented irritability. The state of sleep, which may be viewed as the first 
stage of hybernation, induces an impaired degree of respiration. This would 
soon be attended with pain, if the irritability of the heart were not at the same 
time augmented, so as to carry on the circulation of a less arterial blood, and 
the animal would draw a deep sigh—would augment its respiration, or awake. 
Occasional sighs are, indeed, observed in the sleep of all animals, except the 
hybernating. In these, the circulation goes on uninterruptedly, with a dimi¬ 
nished respiration, by the means of an augmented irritability. There is no stag¬ 
nation of the blood at the heart; consequently, no uneasiness; and the ani¬ 
mal becomes more and more lethargic, as the circulation of a venous blood is 
more complete. This lethargy is eventually interrupted by circumstances 
which break ordinary sleep, as external stimuli, or the calls of appetite. 
Moderate cold disposes to sleep,—to lethargy. But severer cold induces a 
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