OSSICULA AUDITUS IN THR HIGHER MAMMALIA. 900 
called tlie sulcus tympanicus, while the ventral and internal portion opening into the 
pharynx may he termed the tubal portion of the sulcus. This sulcus does not expand 
until after birth, when the foetus begins to breathe air; the walls of it, even in the 
oldest embryoes, being close together, although in some of the figures, owing to the 
obliquity of the sections, they are a little distance apart (Plate 57, figs. 29, 30, 31, 32, 
and 34). 
There is at first no external auditory meatus; it is formed later on by the growth 
outwards of the side walls of the head (Plate 55, figs. 17, 18; Plate 56, fig. 23; 
Plate 57, figs. 29 and 31), leaving a gap leading towards the dorsal part of the hyo- 
mandibular cleft (region of membrana tympani). The growth outwards of the wall of 
the head incloses a median thickening, which contains the ventrally curved extremity 
of the mandibular cartilage (Plate 55, fig. 18); the dorsal region of this growth becomes 
bent upon itself, forming a fold of the integument, which unfolding after birth becomes 
the long dorsally directed pinna of the Rodents (Plate 57, fig. 31). 
The auditory vesicle, after it has been shut off from the external epiblast, has an 
oval form (Plate 54, figs. 2 and 3), and is situated between the hind brain and the 
external epiblast. Yentrad of the vesicles runs the seventh nerve, the primitive 
jugular vein and dorsal aorta. The ganglion acusticum and auditory epithelium are 
very intimately connected, a fact already noticed by Balfour and Marshall (Plate 
54, fig. 2 ; and Plate 55, fig. 17). The ganglion has two well marked divisions, corre¬ 
sponding to the cochlear and vestibular portions of the nerve (Plate 54, fig. 9 ; Plate 
55, fig. 18 ; and Plate 56, fig. 23), although in embryoes near the period of birth it is 
single (Plate 57, fig. 32). 
In slightly older embryoes the vesicle has undergone the ordinary Mammalian 
complications (Plate 55, fig. 17 ; and Plate 56, figs. 20 and 23); the ventral end of the 
vesicle passes inwards towards the base of the skull, forming the canalis cochlearis and 
sacculus hemisphericus; in the hollow of the canal lies the ganglion acusticum ; from 
the dorsal end of the vesicle, on the side nearest the brain cavity, there passes off an 
elongated diverticulum, the recessus labyrinth!, external to which, and running for 
some distance parallel with it, lies the superior semicircular canal. 
The external semicircular canal passes from the vestibular cavity outwards, dorsad of 
the seventh nerve and primitive jugular vein towards the epiblast (Plate 55, fig. 17; 
Plate 56, figs. 20 and 23). The complicated vesicle becomes surrounded by a thickened 
layer of densely packed cells, which follows the outlines of the several parts of the 
vesicle, with the exception of the recessus vestibuli. This layer of cells is laid down 
at the same time as the elementary cartilages in the arches, and the changes by which 
it is converted into hyaline cartilage go on simultaneously with those occurring in the 
cells of the cartilages. 
The points of interest in connexion with the periotic capsule are that it is deficient 
on its internal surface, for the ganglion acusticum and its vestibular and cochlear 
branches ; it is also deficient externally, for the fenestra ovalis and fenestra rotunda. 
