Fore Limb of Frog. 103 



the teeth of the upper jaw. The Jiigah, which extend from 

 the maxillae to the squamosals, are to be looked upon as 

 belonging to the same section of the skull. As to Meckel's 

 cartilage, it is invested by the dentary bone (m.n), which, though 

 a single bone, and thereby differing from the ensheathment of 

 the cartilage in the bird, ossifies from a number of centres, 

 which indicate its primitively compound nature. The number 

 of centres appears to correspond to the number of separate 

 bones in the bird. The second arch is represented proximally 

 by the incus, the stapes, and the os orbiculare — three of the four 

 ear-bones already referred to. The rest of the hyoid arch 

 consists of a median piece, the body, and two pairs of project- 

 ing cornua, of which one is true hyoid, the other a vestige of 

 the'third visceral arch.^ 



The Skeleton of the Fore Limb. 



The skeleton of the fore limb consists of the pectoral girdle 

 and of the limb which articulates with it. 



The pectoral or shoulder girdle itself in the adult Frog 

 is a partly cartilaginous, partly bony structure. Each half of 

 the girdle is C-shaped, the upper end, which does hot meet its 

 fellow, lying above the vertebral column; the lower ends do 

 meet below. The part of the girdle lying above the glenoid 

 cavity, into which fits the head of the humerus, is the scapular 

 region. The uppermost part of this is bent at an angle with 

 the lower part, and is not so much ossified : it is called the 

 supra scapula; the lower, more ossified portion, the scapula. 

 On the ventral side of the glenoid cavity are two bars of car- 

 tilage (in the young frog), which meet at first in the middle 

 ' line ; these are respectively the coracoid and the procoracoid 

 (the most anterior). The extremities of the coracoid and the 

 procoracoid of each side fuse together, the common portion 

 being termed the epicoracoid ;, the two epicoracoids overlap.. 

 On the procoracoid, but independently of it, a membrane bone, 

 the clavicle, is formed, so that in the adult shoulder-girdle it 



' It bas been shown that the cartilages of the larynx are traceable to 

 branchial arches behind these. 



