NATURAL THEOLOGY. 137 



leisure for nothing else. We must have been continually 

 \ipon the watch and continually in fear, nor would this 

 constitution have allowed of sleep." He might have 

 added, we should hardly have performed these func- 

 tions with the exactness and regularity they demand, 

 had they been left to our vigilance and care. 



B. This explains why birds are able to sleep, and 

 yet poise themselves so exactly upon the perch and 

 keep from falling. There is, I suppose, an involun- 

 tary action of the muscles by which they grasp the 

 branch. 



T. This may be partly the explanation, but it is 

 not the whole. There is more mechanical contri- 

 vance for the benefit of the bird. In trussing a fowl, 

 upon bending the legs toward the body, the cook 

 finds the claws close of their own accord. Now this 

 is the very position in which the bird rests, while it 

 roosts upon its perch ; and in this position it roosts in 

 safety. By simply crooking its limbs it produces 

 a contraction of the claws, which remain fast without 

 any voluntary effort as it continues to sit. It is ow- 

 ing to this, the muscle whi^fi pulls the claws to- 

 gether and which comes down the leg for that purpose, 

 is carried round the joints in such a manner in its 

 way to the toes, that it is long enough to reach per- 

 fectly the whole distance only when the animal stands 

 upright, and, therefore, in a" sitting posture, the 

 claws are necessarily contracted. In this way the 

 bird is enabled to roost in safety, even when agitated 

 by the winds. 



M2 



