404 CHORD ATA 



e 



the Hmh, dividing it into dorsal and \cntral portions (fig. ^:o). Th 

 dorsal portion is the scapuUi (shoulder Made) in the peetor;.!, ilium in the 

 pelvie girdle. With the de\ elopment of hone the lower portion is usually 

 split into anterior and posterior parts (fig. sji). The anterior of these is 

 the clavidc in the pectoral girdle, piihic bone in the pehds; the hinder part 

 is the ioracoid or the isc/iiiiiii in the two girdles respectix'ely. These parts 

 are most eoirstant in the ]ielvie girdle. In the pectoral girdle either 

 coracoiil or ela\icle ma\' he lackiirg, at times hoth are ahsent; hut nowile- 

 brate with fore limhs lacks a scapula. In the chnicle there is fre(|uently 

 an element, preformed in canilage, the I'loiorinoid, to he (.listinguished 

 from a memhrane lione, the chnicle in llie strict sense. 



Iir the fishes the girdles are largely or entirely held in position hy 

 muscles; in most terrestrial vertebrates there is a more intimate connexion 

 with th« vertebral column. In the case of the pehic girdle the connexion 

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

 (in reality not with the vertelirx themselves, but bv the inter\'ention of 

 sacral rilis"). The connexion of the pectoral girdle is less direct and is 

 looser. This is elTected Ijv clavicle and coracoid. The latter connects 

 with the sternum, which in turn is connected to the verteliral column by 

 the ribs; the chu'icle articulates with a bone, the cpisUiiuiiii, which rests 

 upon the breast bone, the morphological relation of which is doubtful, 

 since under this term ha\'e lieen included different structures. 



Since only the free portions of the appendages are concerned directly 

 in locomotion, and since the various modes of motion — swimming, fiight, 

 running, leaping, climbing — demand special modifications, the skeleton of 

 the limbs shows great \'arielv. It is usuallv bcliex'cd that all these forms 

 are to be traced back to an ancestral t"\'pe, the orilii/'lcrvi^iiiiji. In this 

 (fig. 5:10) are numerous skeletal jiarts which varv little in si.:e and form 

 and are arranged in many closely appressed rows. Chie of the rows ha 

 acquired prominence and is called the jirincipal row; it begins with a 

 larger piece, the mclaplfryi^liiiii, which articulates with the girdle and 

 bears either on lioth sides {(uu-liiplcryi^iiiiii biscrialt-) or only on one 

 {aniiiph-r\'L;iiiiii iiiiisfridic) the lateral rows of skeletal elements. Usually 

 most of the lateral rows are not attached to the principal row, but arise 

 independently from the girdle, and niav begin with larger parts, the 

 fro pterygium and mcsopU-rygiiiiii. 



From this archipter)gium can be deri\ed a ]M-imar\' form, the penta- 

 dactyle appendage, which serves for all terrestrial vertebrates from the 

 Amphibia onwards (fig. 522). In tracing this from the archipterygium 

 the following modifu-ations must be supposed. First a reduction in the 

 number of rows to five, a principal row and four accessory rows. The 



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