30 



RIBS 



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

 posterior part of the series the two heads often gradually 

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

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

 exceptional in possessing but one head to the ribs, which 

 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 ribs, sometimes called floating rilis, 

 which have no sternal attachment. In the Whalebone Whales 



Fig. 17. — A, First thoracic skeletal segment for comparison witli B, fifth cervical vertebra 

 (Man), b.r, Body of vertebra ; c, first thoracic rib ; c, cervical rib (which has be- 

 come united with the transverse proces.s, tr), the two enclosing the costo-transverse 

 foramen {f.c.t) ; sf, sternum ; zy, articular process of the arch (zygapophysis). 

 {From Wiedersheim's Structure of Man.) 



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

 to which the Whales 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 tlie 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 ribs 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 



