76 On Hybernation. 



pose is not absolutely necessary, as some authors on this sub- 

 ject have supposed. It is not therefore requisite to the ani- 

 mal economy, as in sleep, to prepare it for further exertions. 



The number of animals that hybernate, is much greater 

 than we generally imagine. Dr. Reeve says " the num- 

 ber of hybernating animals is greater than that of those 

 which remain unaffected by cold " p. 5. When the ther- 

 mometer sinks to about 60°, animals that hybernate retire 

 to their hiding places, in trees, rocks, and the earth, where 

 they will be most secure from their enemies. Here they 

 roll tlieinselves up, exposing the least possible surface to 

 the action of the air, and remain in a quiescent state until 

 the return of a more vertical sun warms them into life. Di% 

 Reeve's ideas on the influence of cold on the system are so 

 good I quote them at length. 



'* The cessation of muscular action seencis owing to 

 the lowered temperature of the muscles themselves, 

 because, when the transmission of nervous influence is 

 prevented by dividing the nerve and destroying the 

 brain, the irritability is suspended and recovered exact- 

 ly in the same manner by the operation of cold as in the 

 ordinary state of torpid animals. The loss of motion and 

 sensation, therefore, is owing to the diminished irritability 

 of the muscular fibres, and tiiat again is caused by the ac- 

 tion of cold, and by suspended respiration ; the capillaries 

 of the vascular system appear to become contracted by the 

 loss of animal heat ; and this diminution always begins at 

 the surface of the body and gradually increases to the cen- 

 tre, as observed in examples of numbness from cold, and in 

 applying -the thermometer to different parts of animals 

 whilst they are gradually becoming torpid. We see these 

 animals resist the propenshy to torpor, until by the gradual 

 diminution of their heat and the want of a supply from the 

 absorption of oxygen at their lungs, and at the surface of 

 their bodies, the irritability is so far lessened that it becomes 

 itself a cause of its own deficiency, by arresting the respi- 

 ration, and consequently depriving the heart of its supply, 

 which is furnished by the coronary arteries." — Reeve, 

 p. 55. 



Spallanzani never found the temperature of torpid ani- 

 mals below 36° although exposed to a much greater degree 

 of cold. In this situation, the action of the digestive and 



