380 



THE MECHANICAL FUNCTIONS. 



flying squirrel, and also by some other species of the same 

 genus. They are seen on a still larger scale in the Lemur 

 volans, or Galeopitheciis. The resistance which these broad 

 expansions of skin oppose to the air, when the limbs are 

 spread out, enables the animal to descend in perfect safety 

 through that medium from very considerable heights: but 

 these appendages to the body are mere parachutes, not wings, 

 and none of the animals which possess them can, by their 

 means, and with the utmost efforts which their muscles are 

 capable of exerting, ever rise from the ground, or even sus- 

 pend themselves for a moment in the air. 



The only quadruped that can properly be said to be en- 

 dowed with the power of flying is the Bat. In this animal 

 the portions of the skeleton (f. Fig. 223) which correspond 



to the phalanges of the fingers are extended to an enormous 

 length, and the pectoral muscles, which move the anterior 

 extremities, are of extraordinary size and power. In the 

 larger species, each wing is at least two feet in length. The 

 fine membrane, which is spread between these lengthened 

 fingers, has its origin in the sides of the neck, and reaches 

 all along the body to the extremities of the hinder legs, 

 which it encloses in its folds. Thus, not only is the sur- 

 face, by which it acts upon the air, sufiiciently extensive, 

 but the muscular powder, by which its motions are effected, 

 is adequate to give it those quick and sudden impulses which 

 are requisite for flying: and thus, although its structure is 

 totally different from that of birds, it yet performs fully the 



