24/5 THE SHOULDER GIRDLE. [CHAP. 



Near, or rather below, the middle of the outer surface of 

 this rod is a more or less cup-shaped depression, the glenoid 

 cavity, to which the head of the humerus, or bone of the 

 upper arm, is articulated. The whole rod is separated at 

 this spot into two divisions, which ossify from separate 

 nuclei. The upper or dorsal division is the scapula, the 

 lower or ventral division is the coracoid. 



In all Mammals above the Ornithodelphia the greater part 

 of the coracoid is aborted, but a portion of its upper ex- 

 tremity always remains attached to the scapula as a process, 

 or sometimes as a most inconspicuous nodule, and occa- 

 sionally rudiments of the lower end are found attached to 

 the sternum. The scapula, on the other hand, is always 

 greatly developed. 



A supplementary bone, developed in a different manner, 

 being, at least in the greater part of its extent, ossified from 

 membrane, 1 frequently forms an anterior bar, in front of the 

 coracoid, passing between the scapula and the anterior end 

 of the presternum : this constitutes the clavicle. It is 

 often rudimentary, and very frequently entirely absent, in 

 Mammals. 



Though the scapula may be considered as essentially an 

 elongated rod or bar of bone (a condition most nearly re_ 

 tained in the Mole), it usually has three projecting plates 

 or ridges arranged around its longitudinal axis, and three 

 surfaces or fossae bounded by these, the variations in the 

 extent and form of which give rise to the principal diversities 

 in the form of this bone in different Mammals. 



In the most usual position of the scapula (see Fig. 76), 

 one of these plates (af] projects forwards, this is the 



1 When it has a cartilaginous basis (as has been described by 

 Gegenbaur in Man) this is not a portion of the true primitive shoulder 

 girdle. 



