XXIV] 



SKELETON OF TRUNK 



609 



second and third digits are firmly joined at both ends. The index 

 or second finger has in addition to the metacarpal, in most birds, 

 three other small bones called phalanges, of which the end one 

 sometimes carries a claw : the third digit has only one bone or 

 phalanx besides the metacarpal. The metacarpal of the first 

 digit or thumb is very small, but is like- 

 wise completely fused with the other 

 metacarpals. Besides this the thumb has 

 two joints and often a claw. 



Compared with the arm or fore-leg of 

 other animals the arm of a bird strikes one 

 as having very little flesh. This is be- 

 cause the muscles, especially those on the 

 fore-arm, have comparatively short bellies 

 but very long tendons, in correlation with 

 the often very much lengthened bones, 

 one of which, the ulna, serves as support 

 of the secondaries or arm-quills. 



The movements which constitute fly- 

 ing, namely, the powerful down-stroke of 

 the whole arm and the slower up-stroke, 

 are carried out by the immensely developed 

 pectoral muscles, great fleshy masses which 

 cover the breast-bone or sternum. This 

 bone has a more or less pear-shaped out- 

 line, rounded in front and pointed behind, 

 the ribs ending in its sides (Fig. 304). 

 In accordance with the tendency in all 

 birds to develop the body into a long- 

 neck and a rounded trunk, we find evidence 

 that the number of ribs encircling the body 

 and joining the sternum has been reduced. 

 Not only do we find small free ribs connected with the hinder 

 cervical vertebrae, but attached to the sternum are outgrowths 

 called costal and xiphoid processes (Fig. 304) which are regarded 

 as the remains of sternal ribs the dorsal halves of which are 

 vestigial or lost. If we picture to ourselves the pectoral girdle being 

 thrust backwards and the pelvic girdle pushed forwards so as to crowd 

 the viscera into a small space we shall realise the meaning of the 

 differences between the skeleton of the trunk of a Reptile and that 

 of a Bird, but it must not be supposed that the great length of a 

 s. &M. 39 



FIG. 303. Bones of the right 

 wing of a Gannet, Sula 

 alba x . 



1. Humerus. 

 3. Ulna, 

 metacarpal. 

 metacarpal. 

 or pollex. 

 digit. 8. 



The distal phalanges of 

 the thumb and second 

 digit were wanting in the 

 specimen from which this 

 figure was drawn. 



2. Radius. 



4. Second 



5. Third 



6. Thumb 



7. Second 



Third digit. 



