70 



SKULL OF A 43-MM. HUMAN FETUS. 



of cartilage (figs. 24, 25) which is inclosed by the 

 semicircular canals. A small area in front of the 

 anterior limb of the lateral semicircular canal has 

 failed to chondrify fig. 20, o). It is not seen in 

 later stages. 



The fenestra vestibuli (fig. 20) is roughly oval in 

 contour, with a narrow anterior extremity. Its 

 ventral border, which is formed by the upper free 

 edge of the promontory, is almost straight. The 

 edges are thin, and of a young type of cartilage, 

 which is indefinitely merged with the membrane 

 which fills it. This membrane, which represents 

 t he annular ligament, bulges into the cavity of the 

 capsule, as shown in figure 15. It resembles 

 precartilage and its inner surface is cellular and 

 heavily staining. The stapes occupies only a 

 small fraction of its area. A slender isthmus of 

 cartilage separates the fenestra vestibuli from the 

 fenestra perilymphatica, and from this a low ridge 

 runs directly backward to the lateralmost end of 

 the jugular foramen. 



The fenestra perilymphatica, at the caudal ex- 

 tremity of the cochlea, looks directly backward, 

 and hence can not be shown in the ordinary views, 

 but in figures 5, 6, 17, and 23 arrows are used to 

 indicate its position. Its margin has the form cf 

 an irregular ring, sharply bent upon itself, the bent 

 parts being situated (laterally) at the commissure 

 separating the two fenestrae, and (medially) at the 

 inner corner of the foramen, which will form the 

 cochlear aqueduct. The parts of the ring which 

 approach one another are the posterior intra- 

 perilymphatic process, situated above and pos- 

 teriorly (in my former article referred to as the 

 interperilymphatic process) and the anterior in- 

 traperilymphatic process situated below and 

 interiorly. The latter is a small ridge of cartilage 

 which appears upon the inferior border of the 

 fenestra. It seems to correspond to that described 

 by Terry in the cat, and marked with an asterisk in 

 his figures 2 and 12. These two points are appar- 

 ently growing together, as shown by later stages. 

 In 7a the posterior process was somewhat longer. 

 Thus is accomplished the partitioning of the 

 perilymphatic fenestra into the cochlear fenestra 

 laterally and the cochlear aqueduct medially, 

 The medial half of the border is seen in figure 23. 



Jugular foramen. — The posterior margin of the 

 jugular foramen (fig. 10) has already been de- 

 scribed with the occipital region. The anterior 

 margin, lying at a higher level, is seen in figures 

 15 and 17 between the basicochlear and capsulo- 

 occipital commissures. This border is bent 

 downward by the inferior ampullary prominence 

 and posterior intraperilymphatic process. Be- 



tween this process and the anterior intraperi- 

 lymphatic process is a gap in this border which will 

 be completed by the union of these processes. 



The intracranial ganglia of the glossopharyn- 

 geal, vagus, and accessory nerves are continuous 

 and form a conspicuous object just above the level 

 of the jugular foramen. This chain of nerve cells 

 lies almost directly antero-posteriorly, with a slight 

 medial inclination of the caudal end, and crosses 

 the jugular tubercle, which it almost touches. The 

 jugular ganglion of the vagus is the largest element 

 of the chain; it has the form of a ring, the central 

 part being composed almost entirely of fibers. 

 The outer edge of this ring, situated at a higher 

 level than the inner, lies just medial to the 

 inferior ampullary prominence. In front of this 

 ganglion, and a little above the site of the cochlear 

 aqueduct, is the jugular ganglion of the glosso- 

 pharyngeal. The accessory portion of the chain 

 is long and thin. The petrous ganglion of the 

 glossopharyngeal is situated close behind the lower 

 border of the perilymphatic foramen, its upper 

 limit reaching as high as the anterior intraperilym- 

 phatic process. It is larger and thicker than the 

 jugular ganglion of the same nerve. Between its 

 ganglia the trunk of the glossopharyngeal nerve 

 runs downward and slightly outward just medial 

 to the line joining the intraperilymphatic processes. 

 The ganglion nodosum of the vagus is a much 

 larger structure. It is very much elongated, with 

 attenuated upper end, which reaches to the lower- 

 most limit of the petrous ganglion but lies slightly 

 medial to this. Its main direction is downward, 

 with an appreciable inclination backward. 



The suprafacial commissure is seen in figures 17 

 and 20, and the cut surfaces, after the removal of 

 the commissure, appear in figures 15 and 16. It 

 connects the pars canalicularis just above the 

 region of the recessus ellipticus with the superior 

 border of the cochlea. It is flattened from above 

 downward and is a little narrower below than 

 above. The edges are quite thin. The relation 

 to the facial nerve is seen in figures 15 and 16. In 

 Kernan's specimen the commissure was incom- 

 plete, but it was complete in Lewis's No. 460. 



The tegmen tympani (fig. 20), which overlies part 

 of the developing ossicles, is a well-marked ridge of 

 cartilage projecting forward from the superior 

 ampullary prominence to form the most anterior 

 part of the pars canalicularis. 



The crista parotica, though less prominent than 

 in 7a, is quite conspicuous and forms a ridge lying 

 between the hole (fig. 16, o) in the capsule and the 

 mastoid process. Just above its anterior cud 

 there is a small area for union wth the crus breve 



