60 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



and its appendages are firmly knit together, in the upper part of the trunk of 

 Birds. The vertebra are closely united to each other; and the ribs are con- 

 nected with the sternum by bony prolongations of the latter, instead of by 

 cartilages. This union is so arranged, that the state of expansion is natural 

 to the thorax, whilst that of contraction is forced. The diaphragm is absent 

 among birds, as among Reptiles ; except in a few species, which most nearly 

 approach the Mammalia. But its deficiency is compensated by this contriv- 

 ance, which keeps the lungs and air-sacs always full, except when the Bird 

 by a muscular effort, expels the air from them, in order that they may be re- 

 filled by a fresh supply. By this means, also, the specific gravity of the body is 

 more constantly kept down, than it could have been, if the lungs had been 

 subjected to the constantly-alternating contractions and expansions, which they 

 perform in Mammalia. It is worthy of remark, that the air which enters the 

 bones and the air-sacs, passes through the lungs, both on its entrance and 

 return ; so as to yield to their capillaries all the oxygen which they can take 

 from it, and of which the blood that it has elsewhere met with has not de- 

 prived it. It is only in the lungs, that it meets with purely venous blood ; for 

 they alone receive the branches of the pulmonary artery; the vessels which 

 are distributed upon the respiratory surface of the air-sacs and bones, being 

 a part of the systematic circulating apparatus. Hence we may regard this 

 curious provision, as being partly designed for the aeration of the blood in its 

 course through the system (this, it will be remembered, being the sole mode 

 in which the function is performed in Insects), and partly for supplying the 

 lungs with air, as from a reservoir, during the violent actions of flight. 



38. The articulation of the anterior extremity with the trunk exhibits a 

 peculiar provision for strength and power, which we find in no other Verte- 

 brata. The two clavicles are united together on the central line, forming the 

 furcula or merry-thought ; and the use of this is to keep the shoulders apart, 

 notwithstanding the opposing force exerted by the pectoral muscles in the 

 action of flight. It is generally firm, and its angle open, in proportion to the 

 power of the wings. Besides this bone, there is another connecting the 

 sternum with the scapula on each side ; this is the coracoid bone, which in 

 Man and most other Mammalia is scarcely developed, being merely a short pro- 

 cess which does not reach the sternum. The sternum of Birds usually ex- 

 hibits a very remarkale development on the median line ; an elevated keel or 

 ridge being seen on it, which serves for the attachment of the powerful mus- 

 cles that depress the wings. In the great development of the sternum, Birds 

 have some analogy with the Turtle tribe : which they also resemble in the 

 deficiency of teeth, and in the development of a horny covering to the jaws: 

 but in these, the lateral elements of the sternum are the parts most developed; 

 whilst in Birds it is the central portion which exhibits the peculiarity. From 

 the depth of the keel of the sternum, a judgment may be formed of the thick- 

 ness of the pectoral muscles, and thence of the powers of flight ; in the Os- 

 trich tribe, where the wings are not sufficiently developed to raise the bird 

 off the ground, the sternum is quite flat, as in the Mammalia. The want of 

 flexibility in the trunk is counterbalanced by the length and flexibility of the 

 neck ; the number of cervical vertebrae is very considerable, varying from 12 

 to 23, the highest number being present in the Swan tribe. They are so 

 articulated that the head can be turned completely round, or moved in any 

 direction. The anterior extremities of Birds being solely adapted to sustain 

 them in flight, the posterior are necessarily modified for their support on the 

 ground. They are usually placed rather far back ; but the spine has a posi- 

 tion more inclined than horizontal, so that the weight may not be altogether 

 thrown forwards. The trunk is supported on the thighs by powerful muscles; 

 and there is another series, which passes from the lower part of the spine 



