472 CHORDATA. 
“sternebree.” The anterior end is called the manubrium 
and the posterior end is the xphoid process. The ribs are 
many in number and articulate by a capitulum Jdefween the 
vertebrae and a tuberculum on the transverse process. This 
peculiar articulation of the ribs is explained thus:—In 
certain fossil reptiles the vertebree are double; each has a 
centrum and an intercentrum which are equal in size. The 
rib articulates primarily with the intercentrum by its 
capitulum. In extant reptiles the intercentrum disappears 
and the rib acquires a secondary connection (the tuber- 
culum) with the transverse process ; the capitular attachment 
may then, in some cases, be given up. In mammals the rib 
also acquires a secondary connection with the transverse pro- 
cess, but although the intercentrum disappears, as in modern 
reptiles, the capitular attachment still remains at the spot 
between the centra at which the intercentrum has dis- 
appeared. ’ 
The intercentra are represented in mammals by the zzter- 
vertebral discs which are only very rarely (cf Mole) ossified. 
The cervical ribs are completely fused on to the vertebree 
and are no longer recognisable as such. The ribs in 
Mammalia have an important function in connection with 
respiration. They are moved upon the vertebre by the 
intercostal muscles. When the ribs are raised the cubic 
capacity of the thorax increases and inspiration takes place, 
conversely when they are depressed. This action is sup- 
plemented by the movements of the diaphragm forming the 
posterior wall of the thorax. 
The thorax can be enlarged in two ways. In the dog, 
horse and most quadrupeds the ribs are much bent, and 
they move forward in such a way that the “narrow” chest 
of these animals enlarges laterally, whereas in man the 
sternum is raised and pushed outwards, so that the chest 
is, in this case, expanded vertically. 
:Limgs.—In the mammalian fore-limb the three proximal 
carpals are known as scaphoid, lunare and cuneiform, the 
centrale is often absent, and the distalia are known as 
trapezium, trapezoid, os magnum and unciform, the last 
being the fourth and fifth distal bones fused. There is very 
often another bone, the fisform, usually attached to the 
