528 



CHORD ATA. 



limbs is in its simplest form an arch with right and left halves, 



each half with an articular surface for 

 the limb, dividing it into dorsal and 

 ventral portions (fig. 563). The dorsal 

 portion is the scapula (shoulder blade) in 

 the pectoral, ilium in the pelvic girdle. 

 The lower portion is usually split into 

 anterior and posterior parts (fig. 564). 

 The anterior of these is the clavicle in 

 the pectoral girdle, pubic bone in the 

 pelvis; the hinder part is the coracoid 

 or the ischium in the two girdles re- 

 spectively. These parts are most con- 

 stant in the pelvic girdle. In the pec- 

 toral girdle either coracoid or clavicle 

 may be lacking, at times both are absent ; 

 but no vertebrate with fore limbs lacks 

 a scapula. In the clavicle there is fre- 

 quently an element, preformed in carti- 

 lage, the procoracoid,to be distinguished 

 from a membrane bone, the clavicle in 

 the strict sense. 



In the fishes the girdles are largely 

 or entirely held in position by muscles; 

 in most terrestrial vertebrates there is a 

 more intimate connexion with the axial skeleton and especially with 

 the vertebral column. In the case of the pelvic girdle the connexion 

 is direct, since the ilium is articulated with one or more sacral 

 vertebrae (in reality not with the vertebrae themselves, but by the in- 

 tervention of sacral ribs). The connexion of the pectoral girdle is 

 less direct and is looser. This is effected by clavicle and coracoid. 

 The latter connects with the sternum, which in turn is connected 

 to the vertebral column by the ribs; the clavicle articulates with 

 a bone, the episternum, which rests upon the breast bone, the 

 morphological relation of which is doubtful, since under this term 

 have been included different structures (the membrane bone of 

 Reptiles, episternum in the strict sense, the cartilage bone, the 

 prosternum of the monotremes and the praeclavia of the mammals). 

 Since only the free portions of the appendages are concerned 

 directly in locomotion, and since the various modes of motion 

 swimming, flight, running, leaping, climbing demand special 

 modifications, the skeleton of the limbs shows great variety. It 



Fio. 564. Right half of shoulder 

 girdles of (A) frog, (B) turtle, 

 (C) lizard. (After Gegenbaur, 

 slightly modified.) cf, clavicle; 

 co, coracoid; e, episternum; 

 s, scapula; s', suprascapula; sf, 

 sternum, in C with bases of 

 ribs. 



