472 CHORD AT A, 



"sternebrae." The anterior end is called the manubrium 

 and the posterior end is the xiphoid process. The ribs are 

 many in number and articulate by a capitulum between the 

 vertebrae and a tuberculum on the transverse process. This 

 peculiar articulation of the ribs is explained thus : — In 

 certain fossil reptiles the vertebrae 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 ifiter- 

 vertebral discs which are only very rarely (cf. Mole) ossified. 

 £ The cervical ribs are completely fused on to the vertebrae 

 and are no longer recognisable as such. The ribs in 

 Mammalia have an important function in connection with 

 respiration. They are moved upon the vertebrae 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. 



'" Limbs. — 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 pisiform, usually attached to the 



