THE INTBENAL SKELETON. 1 83 



foramen, called, the foramen ovale^ ia pierced in the. ajisphenoid. 

 It transmits the third branch of the fa th pair of nerves. 



Beneath the sphenoid behind its rostrum is the opening of a 

 passage called the eustachian tube or canal — which bifurcates as 

 it passes backwards to the bony cavity of each ear ; but the 

 two tubes may open separately on the under surface of the skull. 

 Further back, on either side is the opening of another canal, 

 which transmits an important blood-vessel called the carotid * 

 artery. These parts are bounded below by the bony plates 



a, Surangular bone of mandible ; ar, articular bone ; d, dentary ; f, frontal ; 

 i, jugal ; I, lachrymal ■„ me, median ethmoid ; mx, maxillary bone ; p, 

 parietal ; pf, post- frontal process ; pt, pterygoid bone ; px, premaxilla ; 

 5, quadrate bone ; gj, quadrato-jugsd ; sq, squamosal; v, vomer. 



called " basi-teraporals." The nerve which moves the tongue, 

 the ninth or " hypoglossal nerve," passes out through the basi- 

 occipital, and the nerve called " vagus " makesi its exit from the 

 skull more externally and a little anteriorly to the former.. A 

 little more anterior still, is that hinder opening of a canal for 

 the carotid artery just mentioned. 



Part of the interorbital septum and more or less of the 

 median partition between the nostrils may remain gristly or 

 cartilaginous. In the very young condition the skuU is entirely 

 formed of membrane and cartilage, and whien the bones begin 

 to form they are numerous and distinct, but they mostly soon 



* See below, p. 217. 



