26 



eight pairs ; and in the willock, of the order Natatores, twelve 

 pairs; the average number is eight or nine pairs. The true ribs 

 are joined to the sternum by straight osseous portions, called ster- 

 nal ribs, instead of by elastic cartilages, and are movably connected 

 at each end. It is highly interesting to observe the mode of articula- 

 tion of the vertebral extremity of the ribs. In such as require 

 fixity of the chest, for the purposes of flight, the rib is articulated 

 to the side of the vertebra, having no connection to the inter-arti- 

 cular cartilage; but in the ostrich, and others of the cursores, 

 where the dorsal vertebras preserve their mobility, the heads of the 

 ribs are attached to the intervertebral spaces. 



There is another interesting fact connected with the ribs, in 

 birds of powerful flight ; it consists in a number of small osseous 

 plates being detached from the posterior margin of each true rib, 

 and passing backwards and upwards to be connected to the suc- 

 ceeding rib, by means of strong, oblique, fibrous ligaments. In 

 the ostrich, rhea, emeu, and cassowary, these bony splints present a 

 mere rudimentary type. 



Sternum. — The modifications presented by this bone, strictly 

 conform to the functions which the anterior extremities are designed 

 to execute. Hence, in the cursorial birds which do not fly, it is met 

 with in the form of a simple square plate of bone like a shield, while in 

 those birds that possess great powers of flight, it extends over the 

 whole of the lower aspect of the thorax and abdomen, even to the 

 pubic bones, and in order to afford sufficient space for the attach- 

 ment of the pectoral muscles, it is armed with a huge crest, extend- 

 ing the whole length of its lower surface. This crest, or keel, is 

 uniformly developed in proportion to the size of the pectoral mus- 

 cles, and hence declares the power of the anterior extremities. The 

 sternum is sloped obliquely on each side of its anterior extremity, 

 for the reception of the clavicle, and in the centre is connected 

 with the fork-bone, either directly or by means of ligaments; the 

 true ribs are attached to it on each side. The sternum of the crane 

 has within it a large cavity, containing several convolutions of the 

 trachea. This condition is repeated in the wild swan; but in the 

 tame swan, and some of the grallae, as the ciconia and gallinae, it 

 is but very feebly developed. 



On the internal surface of the sternum, chiefly along the mesial 

 line, several apertures may be observed, for the passage of air into 

 the bone. 



Anterior extremity. -—Here, as in the mammalia, the extremity 

 consists of shoulder, arm, fore-arm. and hand. In the first, we 

 recognise a scapula, a coracoid bone, and a clavicle. 



The scapula is generally a long, narrow bone, increasing in 

 thickness as it approaches the shoulder joint, where it expands in 

 the transverse direction, to form the posterior half of the glenoid 

 cavity ; here also it articulates with the coracoid bone and clavicle. 

 The direction of the scapula is longitudinal. In birds with active 



