68 



THE OSTEOLOGY AND MYOLOGY 



pials, as, e.g., the Perameles lagotls, in whicy "the bony roof of the mouth is jjerforated by 

 a wide oval space extending from the second premohrrs to the penultimate molars, exposing 

 to view the vomer and convolutions of the inferior spongy bones in the nasal cavity." 



The remaining portion of the floor of the skull may be here passed over more rapidly, as 

 the most of th? points have been already noticed in speaking of the several bones that com- 

 pose it. Its general contour is that of a lozenge, as far forward as the constricted part, 

 beyond which it enlarges again. The central smooth tract is crossed by the line of separa- 

 tion of the basi-occipital and -sphenoid ; rarely, if ever, obliterated. Just in advance of 

 the most constricted point, a similar, but shorter, transverse fissure indicates the termination 

 of the sphenoid ; this is more liable to disappear by confluence of the following segment. On 

 either side of the basi-occipital, the petrosals determine the extent of the oblique fissurse 

 laceraa. The moderately developed bullte ossete form smooth, convex protuberances just 

 outside the petrosals ; the entocarotid foramen lies immediately anterior, and a little inter- 

 nal ; the foramina ovala et rotunda a little further forward ; the optic still more in advance. 

 The sphenoidal " pterapophj^ses " rise on either side to meet the palatals, partially forming 

 the side walls of the conduit that leads into the posterior nares. These walls are com- 

 pleted by the thin, expanded falciform pterygoids, vertically lodged by their very delicate 

 and attenuated process in a groove in the sphenoid pterapojAyses. The auditory fossae 

 occupy the postero-external corner of the skull, appearing as deep notches between the 

 articular processes of the squamosal and the mastoid, limited internally by the petrosals 

 and sphenoidal bullae. There are two oval foramina, side by side, in the outer part of the 

 notch ; just below them, the delicate tympanic ring is seen suspended at the entrance of 

 the meatus auditorius. 



2. The Other Vertebra. 



a. Cervical Vertehrce. — (Figs. 12 and 13). The neck-bones (which are seven in num- 

 ber, as in other marsupials, and nearly all mammals), form a strong compact pile, so closely 

 locked together that little motion is possible except at either extremity. Fixation of this 



portion of the spine is effected both by large overlapping lateral 

 processes, and by the enormous development of the spines of most 

 of the vertebrae. Extension bej'ond a right line is only permitted 

 at the base of the neck ; twisting is mostly, if not Avholly, confined 

 to the atlas, and only an inconsid- 

 erable amount of flexion is attain- 

 able. Five or six of the vertebrae, 

 indeed, virtually act almost as one 

 bone, and mobility is sacrificed to 

 stability. Nearly all the motions 

 of the neck arise between the first 

 dorsal and seventh cervical, where 



Fig. 12. 

 Three views of the Atlas. 



Fig. 13. 



movement is very free, in compen- Si.\ Cerviea'is, lateral view. 



1 Owen, Comp. Anat. and Ph>s., ii, p. 346. The au- vertebrates— as an approximation toward the defective condi- 

 thor considers the vacuities of the bony palate of marsupials tion of this part of the avian skull. 

 — animals forming the transition from oviparous to placental 



