40 THE FROG 
1. The pectoral girdle almost completely encircles the body a 
_ short distance behind the head. Each lateral half of the pectoral 
girdle consists of two portions, dorsal and ventral, which meet at 
the shoulder joint. The dorsal ends of the two halves are 
attached by ligaments and muscles to the vertebral column, while 
the ventral ends are united together in the median plane by the 
sternum or “breast-bone.” 
(a) The dorsal portion is made up of two parts: 
The scapula, a flat elongated bone somewhat constricted in the 
middle. The postero-ventral angle forms part of the wall of a 
cup-shaped cavity, the glenoid fossa, in which the arm articulates. 
The suprascapula is a broad plate of cartilage fastened to the 
dorsal end of the scapula. When in its natural position its thin 
expanded distal end overlaps the first four vertebrz. In the 
adult frog, its proximal portion is calcified and more or less 
ossified. 
(b) The ventral portion, exclusive of the sternum, is formed 
of two bars: 
The posterior bar, much stouter than the anterior, is formed by 
a single bone, the coracoid. Its outer end forms a large part of 
the glenoid fossa. 
The anterior bar is slender and formed of two elements, clavicle 
and precoracoid, applied closely together along their entire length. 
The anterior element, the clavicle, is a membrane bone. The 
posterior element, the precoracoid, is cartilaginous; in the pre- 
pared skeleton it is usually dried so as to be inconspicuous, but its 
position is indicated by a groove in the posterior side of the 
clavicle. Its outer end forms a small part of the wall of the 
glenoid fossa. 
Compare the scapula, clavicle and coracoid with these elements 
in the skeleton of the turtle, where they form a very regular 
tripod. What is the mechanical advantage of this arrangement 
of parts? Compare the corresponding parts of the human 
skeleton. 
(c) The sternum consists of the following parts, beginning at 
the anterior end: 
The episternum, composed of a nearly circular plate of car- 
tilage and a bony rod, in front of the median ends of the clavicles. 
