44 GENERAL ANATOMICAL CHARACTERS 
In some of the Anomodont Reptiles and Labyrinthodont 
Amphibians these chevrons are attached to the intercentra—or 
imperfect disks alternating with the true centra—which suggests 
that they are primarily intercentral elements which have been trans- 
ferred to the edges of the centra by the disappearance of the inter- 
centra. 
Sternum.—The sternum of mammals is a bone, or generally a 
series of bones, placed longitudinally in the mesial line, on the 
inferior or ventral aspect of the thorax, and connected on each side 
with the vertebral column by a series 
of more or less ossified bars called 
“ribs.” It is present in all mammals, 
but varies much in character in the 
different groups. It usually consists 
of a series of distinct segments placed 
one before the other, the anterior 
being called the presternum or “ manu- 
brium sterni” of human anatomy, and 
the posterior the xiphisternum, or 
xiphoid or ensiform process, while the 
intermediate segments, whatever their 
number, constitute the mesosternum 
or “body.” In the Whalebone Whales 
the presternum alone is developed, and 
but a single pair of ribs is attached 
to it. 
Libs—The ribs form a series of 
long, narrow, and more or less flattened 
bones, extending laterally from the 
ie as ena ide’ Caleta aes sides of the vertebral column, curving 
sternal ribs. ps, Presternam; ms, G@Oownwards towards the median line 
mesos ternunn: rs, xiphisternum ; cy of the body below, and mostly joining 
fie ee Le the sides of the sternum. The posterior 
ribs, however, do not directly articulate 
with that bone, but are either attached by their extremities to 
the edges of each rib in front of them, and thus only indirectly 
join the sternum, or else they are quite free below, mecting no part 
of the skeleton. These differences have given rise to the division 
into “true” and “false” ribs (by no means good expressions), signi- 
fying those that join the sternum directly and those that do not : 
and of the latter, those that are free below are ealled “ floating ” 
ribs. The portion of each rib nearest the vertebral column and 
that nearest the sternum differ in their charactors, the latter being 
usually but imperfectly ossified, or remaining permanently eartila- 
ginous. These are called “costal cartilages,” or when ossified 
“sternal ribs.” 
ms 
