380 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
admit the sinus endolymphaticus from the transverse ductus which 
crosses immediately in front of and below the aperture. (Fig. 8, 
Pl. VI.) Neither the cavwm nor its osseous roof continue back- 
wards as far as the posterior face of the basi-occipital, but the roof 
becomes membranous and is continuous with a thickened patch of 
dura mater which forms the posterior wall of the cavum, and is 
attached to the centre of the exposed upper surface of the basi- 
occipital and to the crest separating the sockets on the upper surface 
of the body of the first vertebra. (Figs. 3,4,5, Pl. VI.) On either 
side of this patch the cavum is continuous by its posterior aperture 
with a diverticulum, the atriwm sinus imparis, resting on the upper 
surface of the exoccipital, bounded medially by the thickened dura 
mater and laterally by the spoon-shaped process of the stapes. The lat- 
ter rests moveably ona thickened cushion of dura mater, which is at- 
tached in front to the notch of the exoccipital referred to above, and is 
perforated by the ligament connecting the stapes with the tip of the 
malleus. Were it not for the stapes and its ligament the atrium 
sinus imparis would be in free communication with the saccus para- 
vertebralis by the so-called apertura externa atrii. As it is the saccus 
has no other aperture of communication with the cranial cavity such 
as exists in Cyprinus,’ and the contained semi-fluid tissue which fills 
the saccus and permits the movements of the malleus is therefore not 
part of the perilymphatic tissue surrounding the brain, nor is it simi- 
lar to the entirely fluid contents of the cavum and atria sinus imparis. 
One or two further points may be noted with regard to the neural 
canal before investigating further the nature of the movements of the 
ossicles. 
The white thickened patch of dura mater which bounds the cavum 
sinus imparis posteriorly is continued back somewhat further than 
the body of the first vertebra, and has important relations to the walls 
of the neural canal. From it an oblique stripe ascends in the wall, 
parallel to and behind the ascending process of the stapes and reaches 
the roof of the neural canal. (Figs. 8 and 8a, Pl. IV.) Behind it 
lie the roots of the third nerve ; the ventral root of the second escapes 
in front of it and in the angle behind the ascending and articular pro- 
cesses of the stapes, while the dorsal root perforates the stripe above 
the tip of the ascending process. That part of the patch which forms 
the medial wall of the atriwm is further connected in front of the 
1 Hasse—loc cit, p. 589. 
