40 PiiOF. G. 13. HOWES AND MK. H. 11. SWINNEIITON ON THE 



Somewhat similarly related to the postero-external wall of the sac is a large 

 quadrangular cartilage, the cxtranasal (ex.), and from this four processes arise. The 

 two anterior of these {ex. 1 & 2) arc united to the ethmoidal bar so as to enclose 

 a fenestra {n.fJ). The two posterior are free, and the third {ex. 3), together with 

 the posterior half of the parent cartilage, disposed at a sharp angle to the external 

 half of this, contributes to the support of the anterior orbital wall, bounding 

 ventrally the ethmoidal foramen {f.e.), which transmits the ophthalmic branch of 

 the fiftli nerve '. 



In addition to these complex nasal supports, there is present at this stage in 

 the nasal floor a sheet of procartilage which, in order to complete our terminology, 

 we name the subnasal. This, to which we shall return, is wholly independent in 

 origin, and by analogy to the extranasal we suspect that this may also arise 

 independently. 



The only remaining changes in the chondrocranium are mainly of the nature of 

 advancing completion of the parts now described. No new elements are added 

 after Stage Q. These perfecting processes, as they may aptly be termed, are most 

 conspicuous at Stages E. and T, which may accordingly be dealt with independently. 



Stage JR. — A very striking feature of this, as compared with the earlier stages, is the 

 straightening out of the ethmosphenoidal region, which at Q was downwardly rotated 

 {cf. PL III. figs. 4 & 8). This straightening process afi"ects most conspicuously the 

 median parts (internasal septum), which now appear as if withdrawn under cover 

 of the extranasal cartilage {ex., fig. 8). 



Equally striking are changes involving the antorbital and otoccipital regions. 

 That taking place in the former is of the nature of extension most markedly dorsal- 

 wards of the interorbital septum {s.i.), by continuation of the process of elevation of 

 the basal ethmoid already alluded to {antea, p. 39). Under the combined influences 

 at work, growth has become more rapid with the ethmosphenoidal than with the 

 otosphenoidal plate ; and ready proof of this lies in the now upward direction of 

 -the third otosphenoidal process {os.^, flg. I), which at Stage Q {os.^, fig. 4) was arched 

 and reached the highest point. Of the fenestrse described at Q (which in life are 

 closed by membrane, except where they transmit nerves) only one has undergone 

 change, viz. that transmitting the fourth cranial nerve (/. 4, fig. 8). This is now 

 complicated, having become continuous with the fenestrae transmitting the third 

 cranial nerve, the jugular vein, and ophthalmic artery — all four being now closed 

 by one continuous membranous sheet. It is clear, on comparison with Stage Q, 

 that the cartilaginous bridges which originally separated these fenestrse {i. e., the fifth 

 otosphenoidal cartilage and its near neighbours) have been absorbed. 



As an accompaniment of these changes, the small cartilaginous bar bounding at 



' Cf. Osawa, 98 % p. 504. 



