378 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



of the notochord does not take place between the fourth and third 

 vertebrae. Nor does it do so between the second and third, because 

 there is only one notochordal plug between the comparatively flat 

 posterior face of the first vertebra and the deep conical hollow of the 

 anterior face of the conjoined second and third vertebra?. 



Dorsally the first vertebral centrum is quite free from that which 

 follows, but ventrally they are suturally united by delicate plates 

 which dovetail into each other on either side of the aortic canal. Also 

 the anterior face of the first centrum is comparatively flat, much more 

 so than the posterior face of the basioccipital bone against which it 

 abuts. The dorsal surface — that which looks into the neural canal — 

 has two sockets, separated by a narrow median partition, (Fig. 3, PI. 

 VI.) In these, rotate freely, the permanently cartilaginous balls 

 which represent the proximal parts of the first neural arches, and 

 which, intact, are the articular processes of the 'stapedes.' It will be 

 observed from Fig. 12, PI. IV., that more cartilage is present in the 

 first vertebra than in any of those which succeed it. Fig. 8a represents 

 the form of the complete stapes. Besides the ' articular' it possesses 

 two other processes, which are merely ossified in membrane ; these 

 are the slender, ' ascending ' process which lies in the neural canal 

 immediately in front of the point of emergence of the second spinal 

 nerve-roots, and the spoon-shaped ' anterior' process which does not 

 form part of the wall of the neural canal, being separated from the 

 spinal cord by a diverticulum of dura mater, the atrium sinus im- 

 parls, (Figs. 12 and 13, PI. IV., and 4, PI. VI.), to the lateral wall 

 of which the spoon-shaped process fits closely. 



It is obvious that the anterior process of the stapes passes beyond 

 the anterior face of the vertebra to which it belongs. It rests upon 

 the exoccipital at the side of the posterior aperture of the cavum 

 sinus imparts, immediately below the foramen magnum (Fig. 5, PI. 

 VI.) and its rounded anterior border fits into a notch on the posterior 

 margin of that bone, which is very distinct in a profile view of this 

 part of the skull. 



Returning to the malleus it will be remembered that its tip also 

 projects in front of the body of the first vertebra. The internal edge 

 of the tip will be found to be connected by a stout ligament whose 

 fibres have a tendinous lustre with the roughened lateral surface of 

 the spoon-shaped process of the stapes. In the ligament is a small 

 bone — the incus — irregularly oblong in the adult, but style-shaped in 



