1470 



BIRDS 



rapidly decreasing in girth, while the feathers are all directed from the head toward 

 the tail. In those birds in which the neck is not unduly elongated the whole con- 

 tour is, indeed, spindle-shaped, and may be compared to two cones placed base to 

 base at the thickest part of the body. It is essential to the exigencies of flight that 

 the centre of gravity should be on the lower aspect of the body, as nearly as pos- 

 sible immediately below the points of suspension by the wings; and, in order to in- 

 sure this, there is the concentration of muscles and other organs in this region, to 

 which some allusion has been already made. Not only are the fleshy portions 

 of the muscles of the legs mainly confined to the upper portions of these limbs, 

 but the muscles which elevate the wings are actually placed on the under instead of 

 on the upper surface of the body. In the breast of a flying bird the great superfi- 



to.re 



s Forehead 



far-cove t*. 

 Crcua 



DIAGRAM OF A BIRD, TO ILLUSTRATE THE ARRANGEMENT OF THE PLUMAGE, ETC. 



cial muscle, known as the pectoralis major, is for the purpose of depressing the 

 wing; beneath this is, however, a second muscle the pectoralis minor of which 

 the function is to raise the wing bone, or humerus. This is effected by the muscle 

 terminating in a tendon, which passes through a pully over the head of the scapula 

 and metacoracoid, and then being attached on the upper surface of the humerus; 

 that bone being accordingly elevated when the muscle contracts. The same ten- 

 dency to the concentration of structures is exhibited by the organ of voice (syrinx) 

 of a bird being placed within the chest, where the windpipe divides into the two 

 bronchi, instead of, as in Mammals, immediately beneath the lower jaw. 



An important external feature in Birds is the frequent presence of a gland 

 termed the oil gland, on the upper surface of the rump, the function of which is 



