86 



SKULL OF A 43-MM. HUMAN FETUS. 



ARYTENOID CARTILAGES. 



The arytenoid cartilages are in a very unde- 

 veloped condition, being represented by paired 

 masses of precartilage of rather indefinite out- 

 lino, investing cores of cartilage of young type, 

 which outwardly grade off into the surrounding 

 tissue. The prccartilaginous arytenoid, as I have 

 modeled it, shows the surfaces and borders of the 

 adult condition. The dorsal surface is slightly 

 concave from before backwards, and even less so 

 from side to side. In its ventral portion the 

 cartilaginous core comes to the surface (fig. 47). 

 This surface shows caudally a continuation of the 

 precartilage over the surface of the cricoid lamina. 

 The medial surface, smallest in area, presents a 

 roughly triangular shape, with the apex craniad 

 (fig. 46). The wider ventrolateral surface is 

 somewhat hollowed out, as in the adult condition. 

 The ventral border is convex downward, pre- 

 senting a blunt processus vocalis; here the core 

 of cartilage shows a corresponding projection 

 (fig. 47). The dorsal border is sharply concave, 



while the lateral border shows an outward con- 

 vexity with a distinct muscular process as the 

 posterior extremity. The entire mass is, as 

 already noted, fused with the upper portion of the 

 cricoid lamina. The cartilage proper, though 

 separated from the cricoid by membrane, is 

 closely applied to it and is to be described as a 

 cylinder having a general caudocranial direction, 

 with a slight medial inclination. Upon its 

 ventromedian aspect, rather nearer its cranial 

 than its caudal end, is a spur of cartilage, the core 

 of the vocal process, projecting downward and 

 inward (fig. 47). The cavum laryngis is very 

 narrow. There are no corniculate and cuneiform 

 cartilages. 



TRACHEAL RINGS. 



The tracheal rings lie parallel with the basal 

 plate and are of a very young type of cartilage. 

 The upper three show fusions with one another. 

 Each describes about half a circle. Only the 

 upper four were modeled. 



MEMBRANE BONES. 



INTERPARIETAL. 



The interparietal part of the future occip- 

 ital bone is represented in No. 886 by two 

 slender osseous spicules, lying one on either 

 side of the processus ascendens (figs. 1, 4, 5), 

 just above the upper border of the tectum pos- 

 terius. The outer ends are somewhat medial to 

 and below the projecting extremities of the 

 parietal plates. Each half is slightly flattened 

 antero-posteriorly and is invested by condensed 

 mesenchyme, as seen in figure 14. This bone 

 could not be modeled in la and is not included in 

 the Hertwig model, so that comparisons with 

 these specimens could not be made. 



PARIETAL. 



The parietal bone is a very fragile reticulum 

 of osseous substance lying in a plane just 

 lateral to that of the parietal plate. It is very 

 small in comparison with the frontal bone. 

 In outline it is roughly oval with the long axis 

 dorsoventral. The bone is very slightly con- 

 cave inwardly and lies in the membranous cal- 

 varium. Its caudal edge does not as yet overlap 

 the cranial edge of the parietal plate, as it does in 

 7a and the later Hertwig model. The peripheral 

 part is but little more delicate than the central. 

 The texture of the bone was too tenuous for 

 modeling, so that it had to be represented by 

 profile reconstruction. The dorsal edge does not 



extend above the line joining the dorsal extremi- 

 ties of the parietal plate and frontal bone. In la 

 the bone has grown enormously upward and in the 

 Hertwig model this growth is even more marked, 

 while the parietal plate has undergone reduction. 



FRONTAL. 



As in the case of the parietal, the outline 

 of the right half of the frontal bone was worked 

 out in plaster of paris and the details of 

 structure were obtained by profile reconstructions 

 in the sagittal and transverse planes. From the 

 side (fig. 5) the bone appears as a roughly rounded 

 plate bent into an outward convexity. The 

 margins are serrated. Arching across this sur- 

 face is a low but distinct ridge which separates the 

 orbital from the frontal portions. This ridge is 

 recognized as the representative of the supraorbi- 

 tal margin and probably the superciliary arch as 

 well. It slants much more sharply backward than 

 in the adult and extends from a projection of the 

 bone in front, representing the medial angular 

 process, to a point some distance behind the dorso- 

 caudal angle of the bone. The caudal limit 

 doubtless represents the position of the future 

 zygomatic process. The zygomatic bone is sepa- 

 rated from this point by a wide interval. This 

 ridge marks the densest part of the bone and 

 appears to be in the position of the original center 

 of ossification, as I have pointed out (Macklin 



