OSSIOULA AUDITUS IN THE HIGHER MAMMALIA. 
907 
give under this heading a separate description of the embryonic history and adult 
condition of the stapes and the artery with which-it is connected, and finally, briefly 
summarise the results which may be drawn from the work. 
Description of the Work. 
First.—The parts entering into more or less close connexion with the ossicula are the 
gasserian ganglion and maxillary and mandibular branches of the fifth nerve, 
the ganglion, trunk, and mandibular (chorda tympani) branch of the facial 
nerve, the primitive jugular vein, the hyomandibular cleft, the meatus auditorius 
externus, the auditory vesicle and its capsule, the dorsal aorta, and its branch 
passing towards the stapes, and lastly the tympanic annulus. 
The gasserian ganglion is situated dorsad of the cleft of the mouth, over which its 
maxillary and mandibular branches are placed. In the earliest embryoes figured 
(Plate 54, fig. 1; and Plate 55, fig. 11) the nerves and ganglia are of large relative size, 
in the older ones they are less conspicuous. 
The mandibular branch of the fifth divides into two parts which come into close 
relation with the cartilage of the arch a little distance in front of its proximal 
extremity, one part runs internal to the cartilage and is joined by the mandibular 
branch of the seventh, the other passes obliquely external to the cartilage (Plate 54, 
figs. 1, 4, 5; Plate 55, fig. 11; and Plate 58, fig. 39). The ganglion of the seventh is 
situated posterior to and near the ventral border of the gasserian ganglion, compared 
with which it is very small (Plate 54, fig. 6 ; Plate 55, fig. 12; Plate 56, figs. 22, 24, 
26 ; and Plate 58, fig. 39). The trunk of the nerve at first runs in an antero-posterior 
direction, lying with the primitive jugular vein, between the canal portion of the 
labyrinth and the dorsal and external portion of the hyomandibular cleft (Plate 54, 
fig. 6 ; Plate 55, figs. 11, 12 ; and Plate 56, fig. 24). At the level of the lower border 
of the periotic capsule it turns almost at right angles to its former direction, turning 
round and passing external to the hyoidean cartilage on its way to the face. 
Where the above change in the direction of the axis of the trunk takes place the 
mandibular or chorda tympani branch is given off; in older embryoes this branch 
passes off the trunk of the nerve after it emerges from under tie cover of the periotic 
process (a process given off from the periotic capsule to join the hyoidean cartilage); it 
is of large size in very young embryoes (Plate 54, fig. 1 ; and Plate 55, fig. 11), and 
passes between the hypoblast and epiblast forming the closed dorsal and external 
portion of the hyomandibular cleft, then runs in a ventral direction and joins the 
internal branch of the mandibular division of the fifth nerve. In older embryoes it 
comes into relation with the cartilages of the first two post-oral arches passing external 
to the hyoidean, internal to the proximal extremity of the mandibular (Plate 54, figs. 
4, 5, 8 ; Plate 55, figs. 10, 16 ; Plate 56, figs. 19, 21, 25 ; and Plate 58, figs. 39, 40). 
