30 



RIBS 



are there ribs with but one, the capitular, head. Tn the 

 posterior part of the series the two heads often gradually 

 coalesce, so that there comes to be but one, the capitular, 

 head. The AVhales also, at least the Whalebone Whales, are 

 exceptional in possessing but one head to the ril)s, \\hich 

 is the capitular. The first rib joins the sternum below, and 

 a variable number after this have the same attachment. There 

 are always a number of ril)S, sometimes called floating ril)S, 

 which ha^e no sternal attachment. In the Whalebone Whales 



c.t. 



Fig. 17. -A, First thoracic skeletal segment for comparison with B, tifth cervical vertebra 

 (Mau). b.i\ Body of vertehra ; f, first thoracic rib ; c\ cervical lib (which has be- 

 come miited with the transverse process, tr), the two enclosing the costo-transverse 

 foramen (f.c.i) ; sU sternum ; zy, articular process of the arch (zygapophysis). 

 (From WMedersheim's Structure of Man.) 



it is the first rib alone which is so attached. As a rule, 

 to which the AVhales mentioned are again an exception, the 

 rib is divided into at least two regions — the vertebral portion 

 which is always ossified, and the sternal moiety which is usually 

 cartilaginous. This is, however, often very short in the first rib. 

 They are, however, ossified in the Armadillos and in some other 

 animals. Between the vertebral and sternal portions an inter- 

 mediate tract is separated off and ossified in the Monotremata. 

 The ril_)s of existing mammals belong only to the dorsal region 

 of the vertebral column, but there are traces of lumbar ribs and 

 also of cervical ribs. In the Monotremata, indeed, these latter 



