rjBS OF VERTEBEATA. 441 



Crocodiliui, These processes are not laid down iu cartilage, but are 

 secondary ossifications. The lumbar portion of the vertebral column, 

 which is fused with the sacrum, has no ribs in Birds, but distinct 

 radiments may be seen on the true sacral vertebra3, so that even 

 here the ilium is not united directly to the vertebra;, but to the 

 rudimentary ribs that are attached to them. Similar radiments 

 may be made out in the Crocodiliui. When the caudal region is 

 well developed, the same structures as those which have been already 

 alluded to as rudimentaiy ribs may be seen to enclose the caudal canal. 



With regard to the connection between the ribs and the vertebra3 ; 

 in the Saurii, Crocodiliui, and Aves, there are generally two points 

 of attachment, for the rib is articulated to the centrum by a 

 capitulum (^), and to the transverse process by a tubercle {a). The 

 posterior ribs gradually get to have only one point of attachment. 



In the Mammalia the cervical ribs have passed completely into 

 the vertebra;, and it is only now and again that we meet with a free 

 rib. The thoracic ribs, which vary in 

 number, are divided into the two above- ■*! 



mentioned pieces, and so show that ossi- ^~^^Lr^Ssi_-^o. 



fication has not equally affected the whole '^^--^^C"^^^^^^^ 



rib, but that a sternal portion remains in y^ — ^^^^^^mi-^^^^-'''^ 



its cartilaginous condition. When this /\/ ^ tMr 



portion is ossified (Edentata, Cetacea) it -y^ 



forms a separate piece ; in Ornithorhyn- j^jg^ 231. Thoracic vertebra of 

 chus, it is again divided in the case of Buteo vulgaris, c Oeutrum 

 the last five ribs : this happens also in of. ^^^ vertebra, s Superior 



. ' ^ ^ spinous process, tr Irans verse 



Mams. process. loEib. a Tuberculum. 



The anterior ribs alone reach to the ^ Capitulum. 



sternum. The hinder ones are either 



connected with the sternal end of the one next in front, or project 

 freely, and so get to resemble the rudimentary forms, to which 

 class of ribs the hindermost ones in the Cetacea do belong ; these 

 are not even connected with the vertebral column. In the lumbar 

 region the ribs are fused with the transverse processes. It is not 

 certain whether the transverse process itself does not represent the 

 rib. The rudiments of the ribs are much more distmct in the first 

 two or three sacral vertebra;, where, as in the lower classes, they 

 are the means of connection between the sacral vertebra and the 

 ilium. In this region they have the form of ventral pieces attached 

 to the transverse processes. Finally, in long-tailed Mammals, the 

 rudiments of ribs having the form of inferior arches occur. The 

 double mode of union, which is genei'ally seen in the case of the 

 cervical ribs, obtains also in the thoracic ribs, but it is single in 

 the case of the hinder ones. 



