156 TYPES OF ANIMAL LIFE 



in various directions across the room in which the experi- 

 ments were made. This sense is believed to be due to 

 an exceedingly delicate power of sensation possessed 

 by the membrane of the wing a power enabling the 

 creature to feel by atmospheric pressure and vibration, 

 the nearness of adjacent objects. 



Certainly if the wing does possess such sensibility the 

 great extent of its surface must intensify it to a high 

 degree. Now, the wing is richly supplied with nerves, 

 while the power of feeling by means of the nerves depends 

 greatly on the amount of blood supplied to them. This we 

 all know by the numbness we can bring easily on in any 

 one of our fingers by tying a string tightly round its root, 

 which causes it, as we say, to "go to sleep," a condition 

 occasioned by depriving its nerves of their due supply of 

 blood. The circulation of that fluid in man and beasts 

 is brought about mainly by the rhythmical contractions 

 of the heart, while this is aided by the elasticity of the 

 arteries, which, though not themselves contractile, have 

 a power, through their elasticity, of propelling the blood 

 which is not possessed by the veins. 



Now, it is a very remarkable fact that the veins in the 

 bat's wing are positively contractile, thus serving in a 

 most exceptional manner to propel the blood, and so, 

 indirectly, augment such powers of sensation as the 

 delicate membrane of the bat's wing may be supplied 

 with. 



There are probably not less than a thousand different 

 kinds of bats, for most likely the species already 

 collected do not amount to half those which will be 

 eventually known to us. No less than four hundred 

 kinds were fully described a dozen years ago by Mr. G. 

 A. Dobson, a naturalist who has especially devoted him- 

 self to the study of these animals. 



