THE APPENDICULA.R SKELETON 49 



a. The hyoid arch. 



i. The columella (Figs. 9 and 10) is formed from the 

 top of the hyoid arch : it is a small rod, partly 

 bone and partly cartilage, the inner end of which 

 is inserted into the fenestra ovalis, an aperture 

 in the outer wall of the auditory capsule ; while 

 the outer end is attached to the tympanic mem- 

 brane rather above its middle. 



ii. The anterior cornu of the hyoid (Fig. 9) is a long 

 slender curved rod of cartilage, attached above to 

 the auditory capsule just below the fenestra ovalis, 

 and curving at first backwards and then forwards 

 and downwards to be attached to the anterior 

 outer angle of the body of the hyoid. 



b. The body of the hyoid is a flat squarish plate of cartilage, 



formed by the fused ventral ends of the hyoid and 

 branchial arches, and lying in the floor of the mouth : 

 short processes are given off from its angles. 



c. The posterior cornua of the hyoid are a pair of stout 



bony processes, diverging from the hinder border of 

 the body of the hyoid. 



B. The Appendicular Skeleton. 



This comprises the limbs and the limb-girdles. As in the case 

 of the axial skeleton it consists at first entirely of cartilage, which 

 becomes afterwards replaced to a greater or less extent by carti- 

 lage-bone. Membrane-bones, i.e., bones developed independently 

 of cartilage, are very rare, the clavicles being the only examples 

 met with in the frog. (Fig. 7, p. 41.) 



1. The Pectoral Girdle. 



This consists originally of two half rings of cartilage, one on 

 each side of the body, which they encircle a short way behind 

 the head : the dorsal ends of the half rings are attached by 

 ligaments and muscles to the vertebral column, while the ven- 

 tral ends are united together in the median plane by the sternum 

 or " breast bone." 



Each half ring bears in the middle of its hinder surface a 

 cup-shaped cavity, which, Avith the first bone of the fore-limb. 



