THE FROG 121 



The endoskeleton consists of the bony and cartilaginous frame- 

 work of the body. It may be divided into the axial skeleton, which 

 includes the skull and the spinal column, and the appendicular 

 skeleton, which includes the framework of the two pairs of ap- 

 pendages (that is, the legs and the girdles which join them with the 

 trunk). The breastbone also may be conveniently studied with 

 the appendicular skeleton. 



The Appendicular Skeleton. The anterior appendages consist 

 of the forelegs and the pectoral girdle. This girdle is formed of a 

 right and a left half, which meet midventrally ; here they enter 

 into a close union with the breastbone, and form with it a bony 

 and cartilaginous ring which almost completely encircles the for- 

 ward part of the trunk. 



Each half of the pectoral girdle supports one of the forelegs 

 and is composed of two portions, a dorsal and a ventral portion. 

 The former portion consists of two skeletal pieces of nearly equal 

 size, the suprascapula and the scapula, which lie respectively on the 

 dorsal and lateral sides of the body. The suprascapula, the dorsal 

 half, is a broad, thin plate which extends upward over the spinal 

 column. Its broad, free, dorsal end is composed of cartilage ; the 

 remainder of it is bone. The scapula is an elongated plate of bone 

 which extends from the suprascapula to the ventral side of the body. 



The ventral portion of the pectoral girdle consists of two bony 

 and three cartilaginous skeletal pieces. The two bones are the 

 coracoid and the clavicle, the former being the larger and the more 

 posterior in position, and extending from the scapula to the mid- 

 ventral line. Joining them and the scapula is an irregular carti- 

 laginous mass called the paraglenoid cartilage, in the hinder side 

 of which is a depression, the glenoid cavity, in which the humerus 

 articulates. Lying along the entire hinder surface of the clavicle 

 is a slender strip of cartilage, the procoracoid ; and at the medial 

 end of the coracoid and clavicle is another narrow strip, the epi- 

 coracoid cartilage, which joins the like cartilage of the opposite 

 side in the median line. 



The breastbone, or sternum, lies in the medial area and consists 

 of two portions, the sternum and the episternum, each of which 

 is made up of a bone and a cartilage. The sternum lies immedi- 



