51 



With regard to the uses of this pneumaticity of the bones the most 

 probable are the following : — 



Ist. The air, by penetrating all parts of the body, secures the perfect 

 oxygenization of the blood, a highly important matter for animals like birds, 

 which undergo violent muscular exertion. 



2ad. The air becoming rarified by the high temperature of the bird's body, 

 the specific gravity of the bird is diminished, and less exertion required to 

 maintain its flght. 



To this use I must again refer in No. 3 section. 



3rd. From the inflation of the body the muscles are enabled to act with 

 firmer purchase and better leverage. 



4th. It is from this arrangement of air- sacs that the singing birds are 

 enabled to prolong their notes. 



Section ii. — Structure op 'VVrNG. 



There is nothing, I think, in the whole range of Zoology which more 

 forcibly illustrates the great truths of unity of design and adaptibility to special 

 purpose which pervade creation, than the wing of a bird. 



For first let us briefly inquire what it is that a wing is required to do, 

 and then let us examine how the vertebrate anterior member is modified to 

 meet those requirements. 



First then, a wing is the instrument by which a bird strikes the air and 

 raises itself from the ground and maintains itself in the air, and also is enabled 

 to progress. 



A wing then must be an insti-ument capable of producing by its strokes 

 an amount of resistance in the air, superior to the entire weight of the bird's 

 body. 



It must also be capable of producing progressive motion as well as upward 

 motion, and it must be most completely under the control of the bird to allow 

 of all those beautiful adjustments which no one can fail to notice and admire 

 in the flight of birds. 



Looking now carefully at the wing we find that, as I mentioned before, 

 the internal structure of the bones and muscles are homologous to the fore-legs 

 or arms of Mammalia, that is the wing is composed of a humerus articulated 

 with the shoulder blades and clavicles or collar bones. 



In birds, however, there is a further provision for the stability of the 

 wing in the shape of the coracoid process of the scapula, which assumes the 

 mportance of a separate bone, and is firmly attached to the breast bone. 



The clavicles also are modified and joined together, forming what is 

 usually called the " merry thought," but which anatomists designate as tha 

 Furculum. This bone forms a sort of spring which prevents the wings pressing 



