STERNUM 5 I 



ribs, distinct in the young, but fusing early with the diapophyses. 

 The synsacrals often are but shghtly modified from the caudal type. 

 The pelvis is articulated by the ihum to the anterior part of the 

 sacrum, while the ischium may be connected with it by ligaments or 

 (Edentates) even be ankylosed. 



The caudal vertebrae are very variable in number. The more 

 anterior usually retain neural arch and spine, and haemal arches on 

 the ventral side. Towards the tip of the tail the vertebrae undergo 

 gradual reduction so that near the end only the centra remain. 



Ribs occur in the cervical and thoracic regions, less distinct in 

 the sacrum. They are moveable only in the thoracic region, while 

 in the neck, as in most other Tetrapoda, they are fused with the 

 centra in the adult, although separate in the young. The cervical 

 ribs are the transverse processes of human anatomy. In the young 

 they have both capitular and tubercular heads articulated to distinct 

 dia- and parapophyses, the only place in mammals where the latter 

 exist as outgrowths from the centra. The sacral ribs are two in 

 number and are fused with their centra in the adult. The thoracic 

 ribs are flat or slightly rounded, have both heads, the upper reduced 

 to a tubercle. They are divided into two groups, true ribs which 

 reach the sternum, with which they are articulated between the 

 sternebrae; and false ribs ending freely between the intercostal 

 muscles. The true ribs, as a rule, have only the proximal part 

 ossified (vertebral rib) that part which joins the sternum (sternal 

 rib) persisting as cartilage, but in Monotremes, dolphins and Xenar- 

 thra the sternal rib is bone. Occasionally an intermediate section 

 occurs. 



Usually the true ribs exceed the false in number, but whales 

 have, at most, but seven true ribs, while in the whalebone whales 

 the sternum is reduced to the presternum which is reached by only 

 the first pair of ribs. Mammals lack an uncinate process on the ribs. 



STERNUM 



The sternum or 'breastbone' is a skeletal structure, preformed 

 in cartilage, lying in the mid-ventral wall of the coelom. In many 

 Ichthyopsida it is connected with the pectoral girdle, and in most 

 Amniotes with both girdle and ribs. It has been regarded as pecu- 

 liar to Tetrapoda and it has been questioned whether the sternum of 

 Amphibia be homologous with that of Amniotes, because the u§ual 



