AVES BIRDS. 593 



the oblique processes, united by articulating surfaces, limit the mo- 

 bility of the neck. 



Although this portion of the spine is very properly designated 

 the " cervical region," we are not on that account to imagine that 

 the vertebrae composing it are unprovided with ribs : on the con- 

 trary, rudimentary costal appendages are generally found connected 

 with their transverse processes, which, in the young bird, are obvi- 

 ously separate elements, although they afterwards become united by 

 anchylosis. 



(659.) 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 accord- 

 ingly everything 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. 



The ribs appended to the dorsal vertebrae may be called the 

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

 rially assist in strengthening that region. Each rib, as in the Croco- 

 dile, presents a dorsal and a sternal portion connected together by a 

 joint : the former are attached to the vertebrae by a double articula- 

 tion, their spinal extremity being furcate; while the latter are articu- 

 lated 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 mus- 

 cular 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 frame- 

 work. 



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 pro- 

 cess, so as to increase materially the extent of surface from which 

 the muscles of the breast take their origin ; but in the cursorial 



