24 THE VERTEBRATE SKELETON. 



attached to the transverse processes of the vertebrae, but a 

 study of their origin in fish shows that they are really the 

 cut off terminations of the ventral arch, not of the transverse 

 processes which are outgrowths from the dorsal arch. In the 

 tail their function is to surround and protect structures like 

 the ventral blood-vessels which do not vary much in size, 

 consequently they meet one another, and form a series of 

 complete ventral or haemal arches. But the trunk contains 

 organs like the lungs and stomach which are liable to vary 

 much in size at different times, consequently the halves of the 

 haemal arch do not meet ventrally, and then the ribs become 

 detached from the rest of the haemal arch. Having once 

 become detached, they are able to shift about and unite them- 

 selves to various points of the vertebra. They frequently, as 

 has been already mentioned, become entirely attached to the 

 transverse process, or they may be attached to the transverse 

 process by a dorsal or tubercular portion and to the centrum 

 or to the ventral arch by a ventral or capitular portion. 



In all animals above fishes the distal ends of the thoracic 

 ribs unite with a median breast bone or sternum which 

 generally has the form of a segmented rod. The sternum is 

 really formed by the fusion of the distal ends of a series of 

 ribs. In many animals elements of the shoulder girdle enter 

 into close relation with the rib elements of the sternum. 



II. THE APPENDICULAK SKELETON. 



This consists of the skeleton of the anterior or pectoral, 

 and the posterior or pelvic limbs, and their girdles. In every 

 case (except in Chelonia) the parts of the appendicular skele- 

 ton lie external to the ribs. 



1. THE LIMB GIRDLES. 



The Pectoral girdle 1 . In the simplest case the pectoral 

 or shoulder girdle consists of a hoop of cartilage incomplete 



1 W. K. Parker, A Monograph of the Shoulder Girdle and Sternum, 

 Ray Soc. London, 1868. 



