SKULL OF A 43-MM. HUMAN FETUS. 



91 



lateral to the upper portions of the malleus 

 and incus. It is prolonged in front into a 

 slender point, and below into the long, spur-like 

 zygomatic process. The squama proper is seen 

 from the side in figure 5. Its lateral surface, 

 quite smooth, shows a very slight convexity in the 

 coronal plane and in the horizontal plane a more 

 marked one, owing to the fact that the cranial 

 end turns medially in such a marked manner. 

 The rounded caudal end is only a short distance in 

 front of the extremity of the short crus of the incus. 

 The upper border, lying but a little below the 

 upper border of the incus, pursues a fairly direct 

 course forward to the level of the root of the 

 zygomatic process, after which it curves rapidly 

 downward. It is slightly serrated behind and 

 markedly so in front (figs. 31, 34). The lower 

 border, showing small irregularities, describes a 

 gentle curve with concavity downward and passes 

 over upon the zygomatic process. Just above 

 the root of the zygomatic process the squama 

 displays a small foramen, and from this point for- 

 ward the plate becomes very narrow and thin and 

 terminates in a slender process, with irregular 

 saw-like edges, which projects downward, for- 

 ward, and inward. It deviates widely from the 

 zygomatic process, forming with this a deep 

 notch. The medial surface of the squama, fairly 

 smooth, and displaying concavities in the frontal 

 and horizontal planes, is separated from the upper 

 parts of the incus and malleus by but a narrow 

 space. 



The zygomatic process is very long and almost 

 straight. It projects downward and forward, 

 tapering gradually to a point which lies close 

 to the tip of the temporal process of the zygomatic 

 bone. This process is not so heavy as that of la, 

 nor is the squama so wide. These differences are 

 even more exaggerated in the Hertwig model. 



GONIALE. 



The goniale, seen in figures 31 and 34 as a sepa- 

 rate bony element, has already been described in 

 connection with the description of the malleus. 



TYMPANIC BONE. 



The tympanic bone (fig. 5) is so slightly de- 

 veloped as to be found only after careful search. 

 It is a very small rounded nodule, consisting 

 mainly of cells with but little ossified matrix. It 

 appears from the side in the angle between the 

 handle of the malleus and Meckel's cartilage, 

 but is just outside the plane joining these struc- 

 tures. Its ventral tip is below and a little lateral 

 to the goniale, while its posterior and larger end 



is below and extends a little medial to that bone, 

 with which its mass of condensed mesenchyme 

 is continuous. It is situated quite close to the 

 lateral surface of the tympanic cavity and just in 

 front of the developing external acoustic meatus. 

 It is not nearly so far on in development as the 

 tympanic of la, and even in the latter there was 

 none of the ring-like form which is so characteris- 

 tic of the new-born condition and which is well 

 seen in the Hertwig model. 



LACRIMAL. 



The lacrimal bone is a very thin, narrow slip 

 of osseous tissue which projects forward from 

 the outer and upper edge of the posterior maxil- 

 lary process (fig. 39), inclosed by a shell of 

 mesenchyme. It measures scarcely 200 micra in 

 length and is rather less than half as wide. The 

 nasolacrimal duct lies just below it, as the figure 

 shows. As we have noted in the description of 

 the ethmoidal region, there is a long and club- 

 shaped projection of cartilage, the paranasal car- 

 tilage, which points downward and forward from 

 the ectethmoidal cartilage and thus forms a sharp 

 notch between itself and the posterior maxillary 

 process within, opening cranioventrally. The 

 paranasal process occupies the position of the 

 future hamulus lacrimalis, the lower salient end 

 of the posterior lacrimal crest, and the notch 

 within it would then become the sulcus lacrimalis. 

 The lacrimal bone undoubtedly comes to occupy 

 the position of the paranasal cartilage and the 

 groove medial to it. As to the exact manner of 

 its growth we do not have much information. 

 It is usually described as arising from a single 

 center, though Thompson (1907, Morris's Human 

 Anatomy) states that not infrequently the 

 hamulus is a separate element. In such a case 

 it seems probable that this center would occur in 

 or upon the paranasal process and would later 

 fuse with the medial center around the lacrimal 

 sulcus. Thompson also states (p. 75): "The 

 hamular process is regarded as representing the 

 remains of the facial part of the lacrimal seen 

 in lower animals." 



VOMER. 



The vomer consists of two separate, very 

 slender strips of bone which lie side by side 

 along the lower border of the nasal cartilage, 

 as shown in figiue 10. They are widest in the 

 middle portion (fig. 9), the ends tapering off 

 to points. Only at the widest point do the bones 

 reach the level of the lower edge of the nasal 

 septum. The anterior ends curve a little upward. 

 The thicker lower borders are closer together than 



