NATURAL HISTORY 209 



" For, ere the beech anrt elm have cast their leaf 

 Deciduous, when now November dark 

 Checks vegetation in the torpid plant, 

 Expos'd to his cold breath, the task begins." COWPER. 



they are designed for cutting or grinding. The relation of the jaw, 

 and the muscular forces by which it is moved, requires a closer 

 examination. 



In herbivorous animals, which have to grind down their food by 

 constant trituration, the jaw is fixed to the skull, so as to allow 

 the former to have a rotatory movement ; but such a movement 

 would be useless to carnivorous animals, where the grinding operation 

 is not required. 



In carnivorous animals the jaw is locked in the cavity of the 

 skull by ligaments, in the same manner as the parts of a hinge are 

 fixed together. The cavity is deep and elongated, and the 

 articulating surface of the jaw-bone corresponds, so that the joint 

 can have only a hinge-like motion. 



627. This is remarkably conspicuous in the eordycles of the lower jaw of the sea- 

 otter. The jaw of the sea-wolf is composed of several pieces, instead of being one 

 entire bone ; and these pieces are connected by ligaments, so that a greater freedom 

 of motion is allowed, and the concussion to the brain arising from the reduction 

 of crabs, muscles, and other shell-fish upon which the animal feeds, is diminished, 

 the jar being broken by being divided over a number of bones. 



628. Why do some animals undergo a state of torpor during 

 the 'winter? 



Because during the winter they cannot produce more heat than 

 is sufficient to raise their temperature from 20 to 26 above the 

 surrounding atmosphere. It follows, therefore, that while in the 

 hottest part of summer their temperature is nearly the same as 

 that of other warm-blooded animals, it falls to a much lower 

 point in the cold season ; and whenever the depression of 

 temperature attains a certain limit, the circulation and respiration 

 decrease in frequency and energy, so that the animal falls into a 

 state of torpor, or lethargic sleep, which continues until the 

 temperature of the atmosphere is sufficiently elevated to re-establish 

 the activity of the "vital functions. 



