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



Bird in the air, but only to propel it through water (as fins would do) by their 

 action on the liquid. On the other hand, in the Ostrich tribe, the laminae of 

 the feather are quite distinct from each other, and no longer form a con- 

 tinuous surface ; so that the feathers more resemble branching hairs. Here 

 the wings are almost or completely absent ; the birds of this tribe being con- 

 stantly upon the ground, propelling themselves by running, and approaching 

 the Mammalia in many points of their conformation. 



48. The bony framework of Birds presents many remarkable adaptations to 

 the same purposes. In the first place it is to be remarked, that the faculty of 

 locomotion is here entirely delegated to the extremities ; and that the skeleton 

 of the trunk must be consolidated, in proportion to the power with which they 

 are to be endowed, in order to afford their muscles a firm attachment ( 33.) 

 Just as the segments of the external skeleton of the Articulata, therefore, are 

 consolidated in Insects, do we find that the vertebral column and its appen- 

 dages are firmly knit together, in the upper part of the trunk of Birds. The 

 vertebrae are closely united to each other ; and the ribs are connected 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. Reptiles possess but a very imperfect mechan- 

 ism for inflating their lungs. Being destitute of a diaphragm, they are obliged 

 to force air into the chest, by a process resembling deglutition ; so that, strange 

 as it may appear, a reptile may be suffocated by holding its mouth open. The 

 diaphragm is absent among Birds, as among Reptiles ; except in a few species 

 which most nearly approach the Mammalia. But its deficiency is compen- 

 sated by this contrivance, 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 diminished than it could be 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 

 deprived 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 systemic circulating apparatus. Hence we may regard this curi- 

 ous 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. 



49. 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 ster- 

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

 and other Mammalia is scarcely developed, being merely a short process which 

 dose not reach the sternum. The sternum of Birds usually exhibits a very 

 remarkable development on the median line ; an elevated keel or ridge being 

 seen on it, which serves for the attachment of the powerful muscles that de- 

 press 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 



