1 68 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



give rise to the muscles of the appendage, and also mesenchy- 

 matous tissue, which becomes transformed in part into the 

 skeleton. This skeleton is, with the exception of the clavicles, 

 preformed in cartilage, the cartilage formation beginning at 

 about the middle of the limb and proceed- 

 ing thence in both directions. 



The skeletons of both pectoral and pelvic 

 appendages are closely similar in structure. 

 Each consists of a skeletal arch or girdle 

 within the trunk, each girdle supporting the 

 skeleton of the appendage. These girdles 

 are known respectively as the pectoral (^shoul- 

 der) and pelvic girdles. 



The pectoral girdle occurs in its simplest 

 form in the lower fishes, where it 

 is a U-shaped arch of cartilage, the 

 "^'^S|ji| bottom of the U crossing the ven- 

 tral surface of the body beneath 

 the skin, the arms projecting up- 

 wards on either side, and the ends 

 being connected by muscles with 

 the vertebral column.^ The skele- 

 ton of the fin is articulated to 

 either half of the girdle, the point 

 of articulation being usually exca- 

 vate, and known as the glenoid 

 fossa. This fossa serves to divide 

 each half of the girdle into a dor- 

 sal or scapular and a ventral or 

 coracoid portion. 



\\ith the appearance of bone 



(ganoids, teleosts) each half of 



the girdle develops two cartilage 



bones, — a scapula, and a second, usually regarded as a coracoid; 



while the two halves of the girdle proper become separated from 



each other. In the dipnoi, ganoids, and teleosts, these are re- 



1 In the skates the pectoral girdle becomes attached to the backbone by means of a 

 so-called suprascapula. In many other fishes it is connected with the skull by a chain of 

 bones. Elsewhere, except in some fossil reptiles, it is free from the axial skeleton. 



Fig. 178. Shoulder girdle and 

 proximal part of pectoral fin of 

 skate (^Raia), g, right half of 

 girdle ; iiis, mesopterygium ; nit, 

 metapterygium ;/, protopterygium; 

 r, radii of fin. 



