32 COMPOSITION OF- STERNUM CHAP. 
brates, the sternum of the Mammalia consists of a series of pieces, 
as many as eight or nine or even sixteen in Cheloepus, of which 
the first is called the manubrium sterni, and the last the ensiform 
cartilage, xiphisternum, or xiphoid process. The latter often 
remains largely cartilaginous throughout life; in fact this is 
generally but not universally the case with that part of the 
breastbone. The most extraordinary modification of the xiphoid 
process is seen in the African species of the genus Manis, where 
it diverges into two long cartilages, which run 
back to the pelvis and then, curving round, run 
forwards and fuse together in the middle line 
anteriorly. These processes serve for the attach- 
ment of certain tongue-muscles. They were looked 
upon by Professor Parker as the equivalents of 
the “abdominal ribs” of reptiles elsewhere non- 
existent among mammals. This view 1s not, 
however, usually held. The manubrium sterni 
is often keeled in the middle lne below; this 
is so with the Bats, which thus approach the 
birds, and probably for the same reason, 7.¢. the 
need of an enlarged origin for the pectoral muscle, 
which is concerned in the movements of flight. 
In many forms this part of the sternum is much 
Fra. 19.—Sternum )roader than the pieces which follow; this is so 
of the Pig (Sus ; a : : 
scrofa). x. ms, With the Viscacha. In the Pig the precise reverse 
Le eaten is seen, the manubrium being narrower than the 
Be nistervase rest of the sternal bonelets. It will be noticed, 
eae vee however, that in this and simular cases there are 
no clavicles. Ribs are attached between the 
successive pieces of the sternum. When the sternum is 
reduced, as it is in the Cetacea and in the Sirenia, it is the 
intermediate part of the series of bones which becomes abbrevi- 
ated or vanishes. The Sperm Whale has only a manubrium 
sterni and a following piece belonging to the mesosternum. 
It is fair to say that the xiphoid process and the rest of 
the sternum have disappeared, since among the Toothed Whales 
a progressive shortening of the sternum can be seen. In 
the Whalebone Whales the sternum 1s still further reduced ; 
the manubrium is alone left, and to it are attached but 
a single pair of ribs. In Balaena, however, a rudimentary 
