THE DBTELOPMENT OF BIBBS. 237 



The skull, in its earlier condition of cartilage and membrane 

 only, is thus conditioned : — 



There is a median mass of eartUage which invests the anterior 

 end of the notochord and forms the base of the cranial cavity, 

 TLe sides of this investing mass extend upwards and meet 

 above, the internal ears being enclosed in these uprising lateral 

 cartilages. Prom the front of the median investing mass, carti- 

 lage extends forwards in the direction of the middle of the upper 

 mandible, and from this a median and two lateral extensions of 

 cartilage mount upwards in the position where we subsequently 

 find the median and lateral ethmoid bones. There is also an 

 extension upwards from the investing mass on either side, in 

 the place vi^here we subsequently find the alisphenoid. Prom 

 the investing mass, four bars of cartilage (on each side) pass 

 forwards or downwards. The first of these lays the foundation 

 of the pterygoid and palatine ; the second becomes a lateral half 

 of the lower mandible. The third and fourth go to construct 

 the hyoid — the thyro-hyals being ossifications in the fourth 

 descending lateral bar of each side. 



The roof of the cranium is completed by membrane only. 



In this membranous and cartilaginous cranium various 

 distinct points of ossification arise and lay the foundation of 

 what at first are separate cranial bones. As these bones grow, 

 they soon meet together, and their lines of junction are the 

 " sutures " of the skull. These still remain pretty distinct in 

 a Ohieken till it is nearly two months old. 



Only in the young, even of Mammals, can we clearly pferceive 

 the distinctness of the three bones which together form the con- 

 tinuously anchylosed " periotic capsule," the names of which 

 are : " prootic," " epiotic," aijd " opisthotic." 



The prootic is the most anteriorly situated of the three, and 

 shelters that one of the three semicircular canals * of the 

 labyrinth which is called the " anterior semicircular canal." 

 It also forms the upper margin of the " fenestra ovalis '' 

 and the whole of the foramen through which the auditory 

 nerve passes to the ear — namely, the " meatus auditorius 

 internus." 



The opisthotic is the most inferiorly situated. It forms the 

 lower margin of the " fenestra ovalis," and entirely surrounds 

 the " fenestra rotunda." 



* See ante p. 221. 



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