30 RIBS CHAP. 
are there ribs with but one, the capitular, head. In the 
posterior part of the series the two heads often gradually 
coalesce, so that there comes to be but one, the capitular, 
head. The Whales also, at least the Whalebone Whales, are 
exceptional in possessing but one head to the ribs, which 
is the capitular. The first rib joms the sternum below, and 
a variable number after this have the same attachment. There 
are always a number of ribs, sometimes called floating ribs, 
which have no sternal attachment. In the Whalebone Whales 
‘Ty 
. 
NY pl Ai 
\ 
Fic. 17.—A, First thoracic skeletal segment for comparison with B, fifth cervical vertebra 
(Man). 6.v, Body of vertebra ; ¢, first thoracic rib; c’, cervical rib (which has be- 
come united with the transverse process, ¢7), the two enclosing the costo-transverse 
foramen (f.c.t); st, 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 the Armadillos and in some other 
annnals. Between the vertebral and sternal portions an inter- 
mediate tract 1s 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 
