718 AVES. 



transverse processes, which, in the young bird, are obviously separate 

 elements, although they afterwards become united by anchylosis. 



(2034.) But if flexibility is thus abundantly provided for in the 

 cervical portion of the vertebral column, it is quite evident that in the 

 thoracic portion of the skeleton, which has to support the framework of 

 the wings, and sustain the efforts of the muscles connected with flight, 

 firmness and rigidity become essential requisites ; and accordingly every- 

 thing has been done to prevent those movements which in the neck were 

 so advantageously permitted. The bodies and spinous processes of the 

 contiguous vertebrae are therefore here firmly consolidated together by 

 anchylosis ; and, moreover, splints of bone, derived from the transverse 

 processes, overlap each other and still further add to the stability and 

 strength of the back. 



(2035.) The ribs appended to the dorsal vertebra may be called the 

 true ribs ; these enter into the composition of the thorax, and materially 

 assist in strengthening that region. Each rib, as in the Crocodile, pre- 

 sents a dorsal and a sternal portion, connected together by a joint : the 

 former are attached to the vertebrae by a double articulation, their 

 spinal extremity being furcate ; while the latter are articulated to the 

 sides of the sternum. A thorax is thus formed, possessing sufficient 

 mobility to perform the movements connected with respiration, but still 

 affording a strong basis to support muscular action ; and in order to 

 give the greatest possible strength, from the posterior margin of each 

 dorsal rib a broad flat process is prolonged backwards and upwards to 

 overlap the rib next behind, so as in this manner to bind the whole 

 together into one strong framework. 



(2036.) The sternum itself is developed in proportion to the enormous 

 size of the three pectoral muscles which constitute the great agents in 

 flight : it is principally composed of the central azygos element before 

 noticed in the Tortoise, which is here remarkably dilated, and in birds 

 of flight prolonged inferiorly into a deep keel-like process, so as to 

 increase materially the extent of surface from which the muscles of the 

 breast take their origin ; but in the cursorial genera, such as the Ostrich, 

 the Emeu, &c., where the wings are not available for flying, the keel is 

 entirely wanting, and the sternum forms merely a kind of osseous shield, 

 covering comparatively a very small portion of the breast. 



(2037.) Whoever considers the position of the hip-joint in the fea- 

 thered tribes, and reflects how far it is necessarily removed behind the 

 centre of gravity when the bird walks, carrying its body in a horizontal 

 position, will at once perceive that the pelvic portion of the spine, 

 having to sustain the whole weight of the trunk under the most un- 

 favourable circumstances, and at the same time to give origin to the 

 strong and massive muscles wielding the thigh, must be consolidated 

 and strengthened in every possible manner, and that even the slight 

 degree of movement permitted in the dorsal region would here be inad- 



