240 THE BIOLOGY OF THE FROG chap. 



partly inclosing, which is a membrane bone, the clavicle. 

 At its outer end the clavicle is bent forward, and applied to 

 a forward projection of cartilage, the acromion, at the lower 

 end of the scapula. The posterior bar, the coracoid. is a 

 stout bone constricted in the middle, and broadly expanded 

 at its inner end ; at its outer extremity it forms a portion of 

 the glenoid fossa. Between the two ends of the coracoid, 

 and extending forward between the clavicles, are the two 

 epicoracoid cartila^^es, which are usually more or less calcified 

 along the ventral side of their line of junction. 



The cpistermun lies in front of the epicoracoids and con- 

 sists of a basal piece of bone and a terminal piece of carti- 

 lage, which has an almost circular expansion at the anterior 

 end. 



The stermDH proper resembles the episternum in general 

 shape, and in being composed of a proximal piece of bone 

 (mesosternum) and a distal expanded piece of cartilage. 

 The latter has a notch in the middle of its posterior margin, 

 which receives the anterior abdominal vein just before it 

 leaves the body wall. Both the sternum and the episternum 

 are capable of a limited vertical movement, especially at the 

 flexible cartilaginous ends. 



In addition to forming a place of attachment for the 

 muscles which move the fore limbs the pectoral arch pro- 

 tects the various internal organs, such as the lungs, heart, 

 etc., and serves to maintain the general form of the body. 

 It also gives attachment to the ventral muscles which draw 

 back the hyoid, and depress the floor of the mouth, and for 

 these purposes the distal ends of the sternum and epister- 

 num are flattened and expanded, their cartilaginous consis- 

 tency enabling them to accommodate themselves to the 

 changes produced in the body wnll. 



The Fore Limbs. — The upper bone of the fore limbs is 



