| OCR Text |
Show 394 MR. W. p. P Y C R A F T O N THE [Mar. 21, narrow synchondrosis widening slightly forwards. Its dorsal border is embedded in a mass of cartilage lodging the prootic, and dividing the exoccipital from the squamosal. Its anterior border looks somewhat upwards and outwards, and is continued dorsalwards into the prootic cartilage, whose free edge forms the posterior wall of the tympanic recess. Seen from within, the exoccipital is more or less flabellate anteriorly, Avith an elongate posterior stem. The inner segment of its convex border abuts against the basioccipital, the outer is bounded by the opisthotic. The stem is bounded on one side by the vagus foramen, on the other by the foramen magnum. The supraoccipital is completely ossified inferiorly and separated from the exoccipital by synchondrosis. Its superior border is as yet very incomplete, deeply concave, with a crenated free edge. Thus a large lambdoidal or parieto-occipital fontanelle is formed. Its dorso-lateral angle joins the parietal, its ventri-lateral the exoccipital, by means of a short, narrow bar ; between these two areas is a wide chink, separating the supraoccipital from the epiotic. The groove lying below this chink is scooped out of the thin plate of bone joining the epiotic to the supraoccipital. The epiotic, seen from Avithout, is represented by a subcrescentic tract of bone, bounded along its inner border, above by a AA'ide chink, and below by the upper part of a deep groove from the supraoccipital. The upper end of its outer border is embedded in the prootic cartilage, its lower end is separated by a thin band of cartilage from the exoccipital. Seen from Avithin, it takes the form of a perfectly free semicircular coil bounding the floccular fossa posteriorly. Its upper and lower ends are separated by cartilage from the prootic. It is fused with the supraoccipital by means of a narroAv plate of bone extending from the posterior border of its inferior end. (Pl. XXIII. figs. 1, 2.) The prootic, from the outside, appears as a broad oblong tract of cartilage lying between the squamosal and exoccipital. Its free border forms the posterior wall of the tympanic recess, and is continuous Avith that of the squamosal prominence. It is bounded posteriorly by the epiotic. The floccular fossa, at this stage, lies in this tract of cartilage, in the angle between the squamosal and parietal above, and the exoccipital and epiotic below. O n the inside, it is bounded by the epiotic behind, and the opisthotic below. BetAveen its junction with the epiotic and tbe opisthotic its border is deeply excavated to form the outer boundary of the floccular fossa. Its supero-lateral border rests upon the lower end of the squamosal, and cuts off this bone from participating in the formation of the brain-case. It is bounded on either side by the parietal (behind) and the alisphenoid (in front); its anterior border is bounded in part by the alisphenoid, and in part by a mass of cartilage lying betAveen this and the basisphenoid, which probably represents tissue into which ossification was destined to spread from the alisphenoid, prootic, and basisphenoid. The |
