MAMMALS OF THE AIR 329 



of an umbrella by keeping stretched out during flight that portion 

 of the membrane which projects beyond the lower arm. The thumbs 

 and feet are not enclosed within the membrane, and consequently 

 are not increased in length. The reason for this will be ascertained 

 later on. 



A further important part of the anatomy is a bony process known 

 as the spur which is found from the heel backwards, and this spur, 

 it is interesting to notice, does not occur ih any other mammal. It 

 serves as a support for that part of the flying membrane situate 

 between the legs and the tail. The membrane is of great elasticity, 

 and as it is highly important for a man-made machine to be well and 

 regularly oiled in order that it may run smoothly, so it is essential 

 that the Bat should be able to maintain the membrane in an elastic 

 condition. Such being the case, the animal is able, by means of 

 fat which is secreted in special glands situated between the nose 

 and eyes, to lubricate the membrane on each occasion a flight is 

 about to be taken. 



Having to undergo much endurance upon the wing in order that 

 at any rate the insectivorous species may procure the necessaries 

 of life, it is essential that the muscles of the Bat should be of great 

 strength, and this we find to be the case, for it has "very powerful 

 thoracic muscles which are attached to remarkably large shoulder- 

 blades and to a high bony crest developed along the median line of 

 the sternum, which thus presents arrangements very similar to 

 those which obtain in birds." There is, however, this distinguish- 

 ing characteristic : whilst the bones of birds are hollow, or, as one 

 might say, pneumatic, like the tyres of the modern bicycle, those of 

 the Bat are not hollow, but, excepting the bones of the shoulder- 

 blades and clavicles, are very thin and yet quite rigid. There are 

 no air-sacs within the body cavity as in birds, so that the Bat has 

 to rely upon other means whereby the weight of the body is dimin- 

 ished, whilst the surface which goes to support the body is much 

 larger in the winged mammal than in the case of the bird. The 

 supporting surface in a bird is formed by the wings and the feathers 

 borne upon them, but in a Bat it extends beyond the legs to 

 the tail. 



Whilst, as is well known, the Bat possesses a remarkable flight, 

 it is not able to carry out the same wonderful evolutions in the air 

 as its companionable rivals just referred to. The flight of the 

 mammal is mostly jerky. It cannot poise or soar like the Skylark, 



