60 TEXT-BOOK OF ZOOLOGY 



Their structure appears, accordingly, strange and peculiar, but on closer 

 examination will be found perfectly adapted to their mode of life and 

 habits. 



A. Structure of Body and Metliods of Locomotion. 



i. How Motion through the Air is effected. 



1. The Bat possesses an Apparatus for Flight. (a) A flying membrane 

 extends on each side between the body, the fore and hind limbs, and the 

 tail, which equals the body in length. When this is unfolded, the animal 

 rests in the air upon a very large surface. It is not as yet, however, carried 

 by the air. This is effected by the constant downward movements of the 

 fore-limbs, each depression of these limbs bringing in its train a con- 

 densation of the atmosphere. The larger the flying membrane, the 

 more easily will the animal be able to support itself in the air, and the 

 more rapidly will it be able to progress. Hence the strikingly large size 

 of the flying membrane. (What kind of bats are probably the fastest 

 and nimblest in progressing through the air ? Compare with swallow 

 and common fowl.) 



(b) With the greater breadth of the anterior portion of the flying 

 membrane is correlated a great elongation of the bones of the arm. 

 The metacarpal bones and fingers are likewise elongated, and, after the 

 manner of the ribs of an umbrella, keep stretched that part of the 

 membrane which projects beyond the lower arm. The thumbs and 

 feet, not being included within the membrane, are not increased in 

 length. From the heel backwards extends a bony process, the spur, 

 not found in any other mammal. It supports the portion of the flying 

 membrane between the legs and the tail. 



(c) In the act of fluttering the bat alternately spreads out and folds 

 together its flying membrane. (When is it spread ? when folded ? what is 

 the object of these movements ? Compare with seal.) The membrane, 

 accordingly, is highly elastic, and in order to maintain it in this condition 

 it is carefully lubricated every time the animal sets out on one of its 

 flying excursions, the fat used for this purpose being secreted by special 

 glands situated between the nose and eyes. 



(d) Clavicles are present, and give firm support to the arms, on whicli 

 the principal work falls during the animal's progress through the air 

 (see p. 11). 



In the true ribs the lower portion, which in most other mammals 

 remains cartilaginous, is completely ossified, whilst the different sections 

 of the sternum coalesce into one single piece. In this manner the walls 

 of the thoracic chamber acquire great firmness. 



(e) For the performance of much work strong muscles are required, 



