THE FROG 59 



membrane bone. The posterior element, the precoracoid, 

 is cartilaginous; in the prepared 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 ele- 

 ments 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, begin- 

 ning at the anterior end : 



The episternum, composed of a nearly circular plate of 

 cartilage and a bony rod, in front of the median ends of the 

 clavicles. 



The paired epicoracoids are two cartilages lying between 

 the median ends of the coracoids and precoracoids. 



The sternum proper, consisting of a rod of cartilage en- 

 sheathed in bone, projecting posteriorly from the epicora- 

 coids, and a somewhat bilobed plate of cartilage comprising 

 the posterior end of the sternum. 



2. The fore-limbs. When the limbs of the frog are ex- 

 tended at right angles to the body with the palms of the 

 hands and the soles of the feet downward (the natural posi- 

 tion for a salamander or a lizard), the anterior sides are 

 called preaxial and the posterior sides are called postaxial. 

 (See Bourne, Comparative Anatomy, Vol. i, Fig. 5.) The 

 same side is preaxial or postaxial whatever the position of 

 the limb. 



With the exception of the small bones of the wrist and 

 ankle, all the bones of the limbs are elongated, with enlarged 



