114 VEETEBRATA. 



573, D). Its ribs are distinguished by their special length, and 

 they are connected ventrally with a system of cartilaginous or 

 bony pieces (sternum) placed in the middle line of the ventral 

 body- wall. Between the head and thoracic region on the one hand, 

 and between the thoracic and sacral region on the other, there is 

 a region the vertebrae "of which are more movable upon one another. 

 The region which connects the head with the thorax the neck 

 (cervical region) is characterised by the greater freedom of move- 

 ment possessed by its vertebrae, on which the rudiments of ribs are 

 retained. The region between the thorax and sacrum the lumbar 

 region (fig. 573, L) is distinguished by the great size of its trans- 

 verse processes, and at the same time by a greater mobility of its 

 vertebrae which are as a rule without ribs. 



Accordingly the trunk of the higher Vertebrates is divided into 

 cervical, thoracic (dorsal), lumbar and sacral regions, which are 

 followed by the caudal region (fig. 573, C). 



The limbs, which have perhaps been derived from lateral folds of 

 the skin or possibly from parts of the visceral arches, present very 

 considerable differences in their form and function. They may have 

 the form of legs and support the body as in terrestrial animals, or 

 serve for flight as the wings of aerial animals, or they may be used in 

 swimming, as the fins of aquatic animals. It can be shown, however, 

 that in every case they are composed of the same essential parts, 

 the variation, suppression, and reduction of which determines the 

 differences between them. 



Just as legs, wings, and fins are homologous organs, so the anterior 

 and posterior pairs of limbs seem to be repetitions of the same 

 arrangement. In both we can recognise the girdle for the connec- 

 tion with the vertebral column, the shaft composed of long tubular 

 bones and the* terminal region. In tracing the development of the 

 extremities G-egenbaur takes the skeleton of the fin of Ceratodus 

 and of the Crossopterygians (archipterygium) as his starting-point, 

 from which, by the reduction of certain regions and the modification 

 of others, the limbs of the higher "Vertebrates may be derived. The 

 pectoral girdle, that is the girdle of the anterior pair of limbs, 

 consists of three paired pieces the dorsal shoulder-blade (scapula) 

 and two ventral pieces placed one behind the other, known as the 

 praecoracoid (with the clavicle) and the coracoid. The pelvic 

 girdle, or girdle of the posterior limbs, corresponds to the pectoral 

 girdle, and is likewise composed of three paired elements a dorsal 

 element attached to the sacrum and known as the ilium, and two 



