SKULL OF A 43-MM. HUMAN FETUS. 



85 



The greater wing, on the contrary, shows a direct 

 continuity of its cartilage with that of the body. 



The lesser cornua are two thin, fusiform bodies 

 of young cartilage covered with precartilage, 

 which are connected by fine strands of membrane 

 with the body of the hyoid below and with the 

 styloid process above. 



The greater cornua, directly continuous with 

 the body, spring backward to meet the superior 

 cornua of the thyroid cartilage, there being a thin 

 strip of membrane intervening. The anterior 

 surface near the body, and the caudal tips, show 

 precartilage. 



THYROID CARTILAGE. 



The thyroid cartilage, too, has acquired most 

 of its adult form, but there are some outstanding 

 differences. We note at once, from the figures, 

 that its upper edge is closely associated with the 

 body of the hyoid and histological examination 

 reveals a continuity of the cartilage. Thus the 

 hyo-thyroid membrane has not developed. The 

 lamina? approach one another so as to form an 

 even, rounded convexity ventrally and a cor- 

 responding concavity dorsally. Thus there is no 

 evidence of the laryngeal prominence, the hyoid 

 body even projecting farther forward than the 

 thyroid cartilage, as figures 5, 6, and 4G show. There 

 is no thyroid notch as in the adult; on the con- 

 trary, if the young cartilage is removed, there is a 

 distinct notch on the lower border of the thyroid 

 cartilage. This, however, is closed by a bridge of 

 young cartilage, and above it there is a small fora- 

 men with precartilaginous borders, where chondri- 

 fication is incomplete. The line of junction of the 

 two laminae, although cartilaginous, is edged with 

 young cartilage. It shows, upon the inner or 

 dorsal surface, just above the foramen mentioned, 

 a low ridge (fig. 47). There is no evidence of an 

 oblique line. An inferior thyroid tubercle is dis- 

 tinctly seen upon the lower border and also a low 

 superior thyroid tubercle, projecting laterally 

 from just below the upper border — rather closer 

 to it than in the adult. The lateral surface pre- 

 sents a shallow concavity, running craniocaudally. 

 The superior border makes a rather sharper angle 

 with the superior cornu than is the case in the 

 adult. The superior cornu is directly connected 

 by cartilage to the lamina, the portion immediately 

 above this connection being more slender than the 

 terminal portion. The extremities lie caudo- 

 medial to the extremities of the greater cornua of 

 the hyoid, but, although they are closely approxi- 

 mated (figs. 46, 47), they show no continuity, 

 being separated by a thin sheet of membrane. 



It is to be noted that the superior cornu is con- 

 tinuous throughout, not interrupted by the lateral 

 hyo-thyroid ligament, as in the adult. The 

 triticeal cartilages are doubtless vestiges of this 

 cornu. The upper extremity is covered with 

 young cartilage. 



The inferior cornu is much shorter than the 

 superior, and rests upon the shoulders of the 

 cricoid, as shown in figures 45 and 47, being 

 separated by a thin sheet of membrane. The 

 lower extremity is edged with young cartilage. 



CRICOID CARTILAGE. 



The cricoid cartilage displays its signet-ring 

 form, the lamina, bearing the young arytenoids, 

 lying between the inferior cornua of the thyroid 

 cartilage. The lamina presents posteriorly a 

 ridge in the midline, and from this the sides slope 

 down to the vertical ridges, which join the posi- 

 tions of the future facets for the arytenoid and 

 inferior cornu and divide the lamina from the 

 arches. At the upper end of the posterior ridge is 

 a small notch, almost completely filled with pre- 

 cartilage, which extends the superior opening of 

 the cricoid here. To either side of this notch are 

 the areas bearing the arytenoids. As figure 47 

 shows, there is a direct continuity of the pre- 

 cartilage investing the arytenoid with that cover- 

 ing the upper portion of the lamina. When, 

 however, this investment is removed, as was done 

 upon the right side in the models (fig. 47), it 

 is seen that the arytenoid is separated from the 

 lamina of the cricoid by a thin strip of membrane, 

 representing the position of the future articular 

 cavity. The part of the lamina bearing the aryte- 

 noid is raised into a tubercle, best seen from the 

 side. 



The upper border, passing forward upon the 

 arch, shows in front of this tubercle a notch and 

 (in front of the region for articulation with the 

 inferior cornu) the lower border also shows a notch, 

 the two serving to constrict the region between 

 the lamina and the arch. In front of both arches 

 there are tubercles, seen in side view upon the 

 upper and lower borders. In front of the upper 

 of these the border descends rapidly, the two 

 lateral borders meeting to form a distinct notch 

 in the midline, flanked by precartilaginous 

 tubercles (fig. 45). In front of the notches upon 

 the lower border the contour descends a little, 

 but there is no notch upon the lower border, as 

 seen from the front. The anterior section of the 

 arch is by far its most slender part. Like the 

 mature cartilage, the inferior opening is rounded, 

 while the superior is oval, showing flattened sides. 



