BIRDS 145 



All the five thoracic vertebrae bear ribs which unite with the 

 breastbone, and each rib in its upper section possesses a back- 

 ward projection, the imcinate process, absent in all mammals. 

 It should also be noted that the posterior neck vertebrae bear 

 small free ribs. 



The breastbone or sternum of a pigeon is exceedingly large, 

 and is in particular distinguished by the presence of a very 

 prominent vertical plate or keel on its under side. The use 

 of this is to give a large surface to which the powerful muscles 

 of flight are attached. A bat's sternum possesses a small keel 

 for a similar reason. 



Skeleton of Fore- Limb. The special function of this being 

 to serve as a firm support for the wing-quills, it is not sur- 

 prising to find that the bones of the hand have undergone 

 a good deal of reduction in number, and also of fusion, the 

 digits being only three, corresponding to the thumb and first 

 two fingers of a mammal. Such a hand skeleton as that 

 described for Man (p. 31), made up of numerous small bones 

 flexibly united together, is very suitable for a limb capable of 

 performing all sorts of complicated movements, but much too 

 elaborate for a mere support, as in the case of a wing. 



The upper part of the limb skeleton, or shoulder -girdle, 

 includes three bones: the dorsal narrow scapula, the rod-shaped 

 ventral coracoid which unites with the sternum, and a collar- 

 bone or clavicle. In the possession of a distinct coracoid, Birds 

 and Monotreme Mammals agree (p. 138). The two narrow 

 curved clavicles are firmly fused together into the familiar 

 " merry- thought " or fiircula, which acts as a spring and keeps 

 the two wings well separated. 



Skeleton of Hind- Limb. The hip-girdle is made up of the 

 same three elements as in a mammal, i.e. ilium, ischium, and 

 pitbis. The ilium is very large and runs both fore and aft, 

 uniting by its inner surface with the long "sacrum", thus 

 affording a very firm support to the body when the bird 

 rests or walks. Both ischium and pubis, especially the latter, 

 are narrow and backwardly- directed, and do not unite with 

 one another in the mid- ventral line as in a mammal (see p. 31). 



The chief peculiarity in the bones of the free part of 

 the leg is found in the ankle, for the tarsus, instead of 

 being made up of a number of small irregular bones, as in 



VOL. I. 10 



