BEASTS OF THE FIELD 23 



upon two legs do so by leaping. The different parts of the 

 fore and hind-limbs, and their several relationships to limbs of 

 different types, will be explained on pages 35 and 44. 



During life these bones are clothed with flesh, and this " flesh/' 

 if carefully examined, will be found to be made up of an enor- 

 mous number of separate bundles, each of which is again made 

 up of a yet greater number of delicate fibres. Further, it 

 will be found that each bundle is connected with at least two 

 separate bones, and hence it is said to have an origin and an 

 insertion. This arrangement is related to the fact that each of 

 these bundles has the power of rapidly shortening or contracting 

 when stimulated by its special nerve, and by this contraction 

 the muscle moves that bone to which it is fixed at its insertion. 

 To the orderly movement of these muscles the power of walking, 

 breathing, eating, and so on is due. 1 



The muscles of the limbs commonly end in " tendons. " That 

 is to say, the muscle tapers off and passes into a different form 

 of tissue, no longer " fleshy " but glistening, white and fibrous, 

 and forming strong, elastic cords (see p. 27). This enables a great 

 economy to be effected, not only in weight but also in bulk. 



The Body Covering. The skin in most mammals is covered 

 with hair; but in many, as will be shown in a later chapter, 

 other and often very remarkable coverings are found. Herein 

 mammals differ from birds, wherein the body is without exception 

 covered after the same fashion by feathers. 



The Heart and Lungs. All mammals breathe by lungs, and 

 have a heart made up of four separate chambers, whereby the 

 blood can be thoroughly purified, which is not the case with the 

 " cold-blooded" reptiles, wherein the heart has but three chambers. 



The heart and lungs in mammals are enclosed within a separate 

 chamber occupying the fore part of the body, or thorax (chest), 

 and this chamber is shut off from the rest of the body cavity by a 

 muscular partition known as the diaphragm or " midriff," an 

 arrangement which is found in no other animals. 



1 Children might well be set to watch the swelling of the biceps muscle of the arm, 

 when the hand is drawn up towards the shoulder ; by way of an object lesson as to 

 the significance of the " flesh " of animals, and as to the way in which this mass of 

 flesh or ' muscle " acts when, by the stimulus of the will, its fibres contract. 



