SKELETON. 7OL 
zygapophyses, below the anapophyses the posterior inter- 
vertebral notches—passages through which the spinal nerves 
pass out, and anteriorly a similar pair of notches. There are 
twelve or thirteen pairs of ribs which support the wall of the 
thorax and aid in the mechanism of respiration. The first 
seven pairs articulate with the breast-bone, the eighth and 
ninth are connected to the ribs in front, the others are free. 
Any one of the first seven or more typical ribs consists of 
two parts, a vertebral portion articulating with a vertebra, an 
imperfectly ossified sternal portion connecting the end of 
Fic. 383.—Side view of rabbit’s skull. 
Pitx., Premaxilla; WVa., nasal; Fr., frontal; Pa., parietal; Sg., 
squamosal; S.O., supraoccipital ; Per, periotic; 7., tympanic 
(the reference line points to the bony external auditory meatus, 
beneath it lies the inflated bulla); O., paroccipital process. 
the vertebral portion with the sternum. Each of the first 
nine ribs has a double head—the capitulum articulating 
with the centrum of the corresponding vertebra, and partly 
with that of the one in front, the tubercle articulating with 
the transverse process of the corresponding vertebra. The 
posterior ribs have no tubercles, and the capitular articula- 
tions are restricted to the corresponding vertebre. 
The sternum is a narrow jointed plate, with a large keeled 
preesternum or manubrium, then five segments composing 
the mesosternum, then a posterior xiphisternum ending in 
cartilage. 
