SKULL OF A 43-MM. HUMAN FETUS. 



75 



edge is of a somewhat younger type of cartilage 

 than that of the lower portion of the plate. 



In front of the dorsum sellae the body of the 

 sphenoid, which here forms the floor of the hypo- 

 physeal fossa, is wide and thin and slightly hol- 

 lowed, but unperforated. Low upon the lateral 

 border appears the thin uninterrupted cartila- 

 ginous union with the alar process (fig. 10). Join- 

 ing the root of this process with the posterior 

 petrosal process is a narrow ridge, edged with young 

 cartilage, forming the lateral border of the caro- 

 tid foramen. 



In front of this thin floor the cartilage narrows 

 transversely and broadens dorso-ventrally as it 

 passes forward. The upper surface presents four 

 distinct elevations in front of the dorsum sella 

 and, including the hypophyseal fossa, four de- 

 pressions or notches between these. The anterior 

 wall of the hypophyseal fossa is slightly hollowed 

 and is surmounted by the tuberculum sellae. To 

 its extremities are attached the stout metoptic 

 roots of the lesser sphenoidal wing (figs. 1, 10). 

 The tuberculum sellae is not so wide as in la, nor 

 is there a middle clinoid process present, as there 

 was on the right side in la. 



In front of the tuberculum sellae the cartilage, 

 now known as the lamina hypochiasmatica, 

 narrows rapidly from side to side and descends 

 into the sulcus chiasmatis. The upper surface is 

 slightly convex. To the lateral margins are 

 attached, just in front of the caudal roots of the 

 lesser wing (with which their bases are continu- 

 ous), the alae hypochiasmaticae. 



ALA HYPOCHIASMATICA. 



These projections seem to be homologous with 

 the orbital processes of Terry, in the cat. They 

 are thin, wing-like structures of young cartilage 

 with rounded borders of precartilage which project 

 outward and a little downward into the optic 

 foramina. Thus each forms with the anterior 

 margin of the posterior root, a notch opening 

 outward into the optic foramen. Anteriorly each 

 wing is separated from the posterior end of the 

 prechiasmatic commissure by another notch — also 

 an extension of the optic foramen. 



The ala hypochiasmatica of No. 886 is less de- 

 veloped than that of la. In relation to the orbital 

 wing it is a very insignificant object in both stages 

 and it is therefore of interest that Kernan (1916) 

 has reported this structure in his 20 mm. 

 human embryo as relatively very large, when 

 compared with the lateral part of the orbital wing. 

 He describes it as a free cylinder of cartilage in the 

 position, apparently, of the future posterior root of 



the orbital wing, to which he considers it to belong. 

 Lewis, in the 21-mm. stage, describes the same 

 structure as the "basal part" of the orbital wing. 

 Here, too, it is relatively very large when compared 

 with the lateral part of the wing. Lewis thinks 

 that the basal part "must ultimately become in- 

 corporated into the body of the sphenoid." From 

 a consideration of these different stages it would 

 seem that the ala hypochiasmatica is very pre- 

 cocious in its development, early attaining a 

 prominence which is not maintained in later life. 

 It is possible that the ala hypochiasmatica, as 

 described by me in No. 886 and la, does not 

 represent all of the structure described by Kernan 

 and Lewis. It would be of interest to ascertain 

 how far the development of this structure is con- 

 cerned with the attachment of the muscles of the 

 orbit. Fawcett (1919) finds, in the chondro- 

 cranium of the bat, all the ocular muscles except 

 the two obliques attached to the hypochiasmatic 

 wing, and Lewis (1920) reports a similar finding 

 in his human embryo. Voit finds an independent 

 ossification center in the ala in the rabbit chon- 

 drocranium. 



PRECHIASMATIC COMMISSURES AND FORAMINA. 



Connected with the edges of the hypochiasmatic 

 plate (figs. 1, 10), lateral to the sulcus chiasmatis 

 and a little in front of the hypochiasmatic wings, 

 arethecaudalmost extremities of the prechiasmatic 

 commissures, slender strands of precartilage with 

 extremities of young cartilage, which extend for- 

 ward to the attenuated anterior roots of the lesser 

 wings of the sphenoid. They are almost negligible 

 when compared with those of la and are evidently 

 just developing. 



These commissures cut off the small prechias- 

 matic foramina from the optic foramina. Fawcett 

 (1918, p. 425), on the basis of his researches with 

 the chondrocranium of Weddell's seal, ferret, and 

 cat, prefers to think of the prechiasmatic foramen 

 as an isolated part of the orbito-nasal fissure 

 rather than as a cut-off part of the optic foramen. 

 These foramina are oval in shape, with long axes 

 directed caudocranially, with a slight deflection 

 of the caudal extremities laterally. Underlying 

 them are the upper extremities of the ectethmoids. 

 They were present in la, being a little farther 

 apart and relatively smaller. They are also 

 present in the Hertwig model. 



In my former article I stated (p. 390), that the 

 prechiasmatic foramina are not to be found in the 

 osseous condition. This statement should be 

 revised, for, although in some skulls they are not to 

 be found, they may persist in others. I have 



