SKULL OF A 43-MM. HUMAN FETUS. 



71 



of the incus. Its posterior portion is concerned 

 with a union with the styloid process (figs 5, 6, 

 16). The facial nerve courses under the shelter 

 of the ridge (fig. 16) in the site of the future facial 

 canal. 



The styloid process (figs. 4, 5, 6, 42, 44) is 

 attached to the otic capsule, thus differing from 

 the condition in la, where it was separate. In 

 figures 42 and 44 of the process and figure 20 of the 

 capsule the division has been made higher up than 

 one would think should be the case, judging from 

 figure 16, and it would seem that a little of the 

 crista parotica has been excised and appears as the 

 upper expanded end of the process in figures 42 and 

 44. There is along this line of section, however, a 

 layer of lighter-staining cartilage which was used 

 as a guide in making the division. At the 20 mm. 

 stage Kernan states that the styloid process is 

 "received in a depression of the cartilage" at the 

 dorsal extremity of the crista parotica. 



The first part of the process passes almost 

 directly inward, with a slight inclination down- 

 ward, and approaches quite close to the capsular 

 wall, which here rises slightly as a very low narrow 

 ridge. Thus we have almost a complete primitive 

 stylo-mastoid foramen for the exit of the facial 

 nerve, the relation of which to the styloid process 

 appears in figures 42 and 44. The styloid next 

 sweeps downward around the posterior edge of the 

 developing tympanic cavity, forming a curve with 

 a strong backward convexity. It then passes 

 into the terminal long, tapering, and almost 

 straight part which is directed downward, inward, 

 and forward. It shows a membranous connection 

 with the lesser cornu of the hyoid (figs. 42 and 44). 



The facial nerve, after passing around the styloid 

 process, turns downward and then forward, and 

 breaks up into the usual branches (fig. 42). From 

 the lateral aspect of the concavity thus made, and 

 but a short distance below the root of the styloid, 

 the chorda tympani is given off (fig. 16), which 

 passes almost directly forward, just above the 

 superior extremity of the tympanic cavity and 

 between the crus longum of the incus and the 

 manubrium of the malleus, after which it turns 

 downward and runs also a little forward and in- 

 ward, crossing the line of the tympanic cavity 

 and tuba auditiva, but diverging from these to 

 reach the lingual, as shown in figures 31 and 34. 



The mastoid process of the right side is a small 

 nodule of young cartilage separated from the cap- 

 sule by perichondrium. Its shape is that of a short 

 rod, about 300 micra long antero-posteriorly. The 

 nodule on the left side is similar. In la the 

 process was connected with the otic capsule by 

 cartilage. In No. 460 it was somewhat longer, 



re'atively, than in No. 886, but there it consisted 

 of blastema. Lewis found, in other embryos of 

 about the same stage as No. 460, that a separate 

 cartilage in the blastema of the mastoid process 

 was present. He notes the attachment of the 

 digastric and stapedius muscles to the process. 

 Kernan notes the process in his specimen. 



Lewis describes the "mastoid cartilage" to which 

 the mastoid process is attached. In his models it 

 is a flange of cartilage which is attached to the 

 caudal and dorsal border of the pars canalicularis. 

 In No. 886 the mastoid process is attached to the 

 pars canalicularis itself, a short distance in 

 advance of the posterior end of the posterior semi- 

 circular canal. It would seem that part, at least, 

 of this mastoid cartilage of Lewis is later taken up 

 into the caudal end of the pars canalicularis. In 

 No. 886 the region corresponding to the mastoid 

 cartilage of No. 460 is very much less prominent. 



Spiral septum. — Looking into the interior of the 

 cochlea (fig. 15) the spiral septum is seen. In my 

 former article it was referred to as the spiral 

 lamina. It springs above from the posterior edge 

 of the roof of the cochlea (which also forms the 

 floor of the facial canal, as in fig. 15), and shows 

 here a slender connection posteriorly with the 

 adjacent capsular wall, just medial to the im- 

 pression for the recessus ellipticus. In this way is 

 formed the transverse crest (fig. 17). Terry 

 describes a similar formation in the cat. Just be- 

 low this union is a short spur of young cartilage 

 (fig. 15) which projects backward under the 

 anterior extremity of the sacculus. 



This medial part of the septum is much the 

 highest; it diminishes rapidly in height as it 

 passes downward and forward (fig. 15). It makes 

 a conspicuous impression upon the cast of the 

 cochlear cavity, as seen in figure 21. As yet the 

 entire septum shows scarcely one turn, and the 

 second (outer) portion of this (rather less than one- 

 half) is not at all prominent; it gradually becomes 

 obliterated in the depths of the cave which is thus 

 cut off. The edge is of a young type of cartilage, 

 which is evidently growing rapidly. 



The first portion of the cochlear canal space lies 

 medial to the high first portion of the spiral 

 septum, and upon the corresponding region of the 

 medial wall of the cochlea there is a very low ridge 

 (fig. 23), also edged with young cartilage. This is 

 seen in the model of the space (fig. 18) as a shallow 

 groove which, beginning caudad above this 

 perilymphatic foramen, is continued forward and 

 finally forward and upward around the medial 

 acoustic meatus to lose itself in front of this 

 opening. This ridge is less developed than in la, 

 where the posterior portion of it was referred to as 



