THE PECTORAL GIRDLE. 55 
the axial skeleton it consists at first entirely of cartilage, which 
becomes afterwards replaced to a greater or less extent by car- 
tilage-bone. Membrane bones, z.¢, bones developed indepen- 
dently of cartilage, are very rare, the clavicles being the only 
examples met with in the frog. (Fig. 7.) 
1. The pectoral girdle. 
Consists originally of two half rings of cartilage, one on each 
side of the body, which they encircle a short way behind the 
head ; the dorsal ends of the half rings are attached by 
ligaments and muscles to the vertebral column, while the ven- 
tral ends are united together in the median line by the sternum 
or “breast bone.” 
Each half ring bears in the middle of its outer surface a cup- 
shaped cavity, which forms with the first bone of the fore-limb 
the shoulder joint. The part of the arch above the joint is the 
scapular portion; and the part below, which is divided into 
anterior and posterior portions, the coracoid. 
a. The scapular portion: divided into 
i, The suprascapula: the upper portion: a thin 
expanded plate of cartilage overlapping the first 
four vertebre ; partly calcified and partly ossified, 
though very imperfectly. 
ii, The scapula: an oblong bony plate, constricted in 
the middle: it forms the upper half of the 
glenoid cavity, the cavity of the shoulder joint. 
b. The coracoid portion: forms the lower half of the glenoid 
cavity; it is divided into anterior and posterior portions, 
separated from each other by the coracoid foramen. 
i. The precoracoid: a slender horizontal bar of car- 
tilage connecting the anterior edge of the scapula 
with the sternum. 
ii, The clavicle: a slender membrane-bone, closely 
applied to the anterior border of the precoracoid : 
its outer or scapular end is bent forwards almost 
at aright angle. Noteworthy as being the only 
membrane-bone in the appendicular skeleton. 
