BONES OF THE AEM. 



143 



wards before the glenoid cavity ; the scapula is articulated 

 by the smooth face of the acromion process (15), to the 

 clavicle ; and affords an extensive attachment to the muscles 

 of the shoulder and those belonging to the arm and fore-arm ; 

 this bone is present in all animals possessing anterior members, 

 although its form undergoes many changes in birds and rep- 

 tiles. Fig. 145 represents the posterior view. Fig. 146, the 

 anterior view. Fig. 147, a profile of the scapula. 



[ 271. THE CLAYICLE, so called from its resemblance to 

 an ancient key, is divided into a Pi gt 143. 



body, two extremities, two arti- 

 cular surfaces, and two processes. 

 Its shape is that of a small Italic 

 /, placed horizontally ; its inner 

 or sternal extremity (1) is very 

 large, and irregularly cylindri- 

 cal ; upon its point is a large 

 articular surface (2), by which it 

 joins with the interarticular car- 

 tilage placed between it and the sternum ; the round arched 

 body expands and forms the scapular extremity (4), having 

 on its under surface a tuber (5), for the attachment of liga- 

 ments, and upon the outer extremity a plain articular sur- 

 face (6), by which it is united to the acromion process of 

 the scapula. The principal use of this bone is to keep the 

 shoulders apart, and complete the resistance of the scapular 

 arch in those animals, as the quadrumana and rodents, that 

 use their anterior members as prehensile instruments, and in 

 the bats and birds, whose anterior members are organs of flight ; 

 as the down-stroke of the wing tends to force the humerus 

 inwards ; in birds, likewise, the coracoid bone appears as a 

 distinct element of the arch. 



[ 272. THE HUMERTJS (fig. 149) is the homologue of the 

 femur, and, like it, is formed of a head, neck, body, and con- 

 dyles. The large round head (1) is received into the shallow 

 glenoid cavity (fig. 147,4), by which great freedom of motion 

 in ail directions is obtained ; the neck (5) is short and thick, 

 and the body (6) appears as if the upper part were twisted out- 

 wards, and the lower part inwards, the outer side of the body 

 presenting a rough surface (9) for the attachment of muscles. 

 The lower extremity of the shaft is enlarged to form a pulley-like 

 surface, upon which the ulna moves in one plane ; the outer 



