THE SKULL. 66i 



A typical lumbar vertebra will show the centrum and its 

 epiphyses, the neural arch and neural spine, the transverse 

 processes, the anterior and posterior articular processes or 

 zygapophyses, the median ventral hypapophysis, the small 

 anapophyses from the neural arch below the posterior 

 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 wliich 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 

 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 vertebrae. 



The sternum is a narrow jointed plate, with a large keeled 

 presternum or manubrium, then five segments composing 

 the mesosternum, then a posterior xiphisternum ending in 

 cartilage. 



The Skull. 



The skull consists, as in all the higher Vertebrates, of two 

 sets of bones, — cartilage-bones preformed in the cartilage of 

 the original gristly brain-box and its associated arches, 

 membrane bones developing in the investing membrane and 

 not preformed in cartilage. (The names of the membrane 

 bones are printed in italics.) 



We have already noticed the chief characteristics of the 

 mammalian skull, such as the usual persistence of sutures, 

 the two condyles, the bony palate, the fusion of the periotic 

 bones, the articulation of the mandible with the squamosal, 

 the fusion of the parts of each ramus of the mandible into 

 a single bone in the adult, and the three ossicles of the 

 ear. 



