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 cavum 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 atrium 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 on a 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- 

 vertebral 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, 1 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 imparts 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 atrium is further connected in front of the 



1 Hasse—loc cit, p. 589. 



