1899.] osiJHOLOGY or the tubinabes. 395 



trigeminal foramen lies above the internal and auditory meatus, 

 between the prootic and alisphenoid. 



The opisthotic, seen from without, is largely cartilaginous. 

 Ossification has, however, begun in the shape of a crescentic 

 nodule of bone lying immediately below the fenestra ovalis and 

 above the vagus foramen. 



Inside it serves to divide the exoccipital and prootic. Ossi- 

 fication has begun from two centres — caudad behind the vagus 

 foramen, laterad of the extreme upper end of the exoccipital and 

 at the base of the epiotic ; and anteriorly, as a small nodule in 

 the region corresponding with a line continuing the exoccipital 

 suture outwards to the postero-A^eutral border of the prootic. 

 Both ossifications have fused with the prootic, but remain separate 

 from the exoccipital. 



The hasisjilienoid is not visible from without, being concealed by 

 the basitemporal plate. Inside it is bounded, caudad by the 

 basioccipital, laterad by a tract of cartilage which divides it from 

 the alisphenoid, and in the middle line, in front, by the cartila- 

 ginous presphenoid. It forms, with the basioccipital, the anterior 

 region of the metencephalic fossa. The pituitary fossa is lodged 

 in its anterior border, and is yet only a shallow depression with 

 an aperture in its posterior border for the internal carotid canal, 

 k mass of diplce divides the basisphenoid from the parasphenoid 

 below. 



The alisjjhenoid is more or less quadrate in form — viewed from 

 without. Its outer border is convex and received into the concave 

 border of the squamosal (PI. XXIII. fig. 1). It is bounded above 

 by the orbital plate of the frontal, below by the lateral wing of the 

 parasphenoid (p. 384) and the trigeminal foramen. 



The orhitosphenoid is represented only by a sheet of membrane. 



The presphenoid is still cartilaginous and continuous in front 

 wdth the mesethmoid. 



The mesethmoid is a vertical linguiform plate resting upon the 

 distal end of the parasphenoid below and underlying the nasals 

 above. Its posterior border is semicircular, its anterior slopes 

 obliquely backwards and terminates at the free end of the nasal 

 process of the premaxilla. 



The olfactory cavity, seen from the inside, after the removal 

 of the mesethmoid, contains a large posterior and an elongated 

 ventral accessory turbinal. 



The quadrate does not difi'er materially from that of the adult. 

 The otic and squamosal processes are somewhat less distinctly 

 marked. 



The columella has well-marked extra- and infrastapedial rays ; 

 the suprastapedium is very short. 



The articulare can still just be distinguished as a separate 

 element (fig. 1, p. 392). 



Tlie Memhrane-hones. 

 The ixirietal is trapezoid. Its outer, anterior, and mesial 



