OEGAN OF THE SILUEOID HYPOPHTHALMUS. US 



structures which they carry, have been telescoped, as it were, on to the vxpper surface of 

 the basioccipital,- and are thus partly to be met with within the foramen magnum. 



As Fig. 12 represents a section falling through the plane where the first spinal nerve 

 pierces the style-like process of the exoccipital, it will be recognized that we are still 

 within the cranial cavity. The incus is to be seen on each side intervening between the 

 anterior end of the malleus and the spoon-bowl shaped process of the stapes. Although 

 the sacculi are still to be met with behind this plane, this is the last section in which the 

 lagensB cochleœ occur. 



It will be recognized that the fenestration through which the vagus escapes is widely 

 in communication at this point with the fossa lodging the posterior semicircular canal, 

 and that behind this level the two main branches of the vagus, the ramus lateralis and 

 the main triink, diverge (Figs.,13 and 14), the one to reach the upper surface of the fovirth 

 transverse process (Figs. 15 and 16), the other to be distributed to the gill arches and the 

 intestine. The ramus lateralis escapes first into the interval between the epiotic and ex- 

 occipital, an interval which can be traced also further forward (Figs. 11 and 10). and 

 into -which part of the dorsal musculature is continued. In Figs. 11 and 12, are seen some 

 of the great cutaneous branches which come off from the neuromastic canal in the interval 

 between the pterotic and epiotic processes of the supraclavicles, and in the roof of the 

 foramen magnum the persistent cartilage already referred to, which intervenes between 

 the supraoccipital and the roof of the neural canal. Above the cartilage is the suture 

 which indicates the overlapping of the neural spine on the supraoccipital, which spine is 

 developed in Amiurus from the region of the third vertebra. 



Six sections intervene between the escape of the first spinal nerve and that of the 

 second. I have only beeii able to find the ventral root of the latter, a circumstance which 

 I am inclined to explain by the reduction of the vertebral segments concerned, This root 

 escapes outward through the saccus paravertebralis which, in consequence of the for- 

 ward dislocation of the part coucerned, rests chiefly on the upper surface of the exoecipital. 

 (Figs. 13 and 14.) An important difference is to be noted in Fig. 13, the section rej)re- 

 sented in which falls through the hinder part of the spoon-shaped process of the stapes. 

 This comes in relation to the atrium sinus imparis which is constituted by the median 

 partition of dura mater attached to the upper surface of the centrum of the first vertebra. 

 The mode in which this centrum has been wedged in between the ex- and over the basi- 

 occipitals is very singular, but persistent cartilage indicates the separation. The median 

 partition of dura mater also forms the floor and part of the lateral walls of the neural canal. 

 It is simply the unossified backward continuation of the ossified roof of the cavum sinus 

 imparis, and a rudimentary claustrum is to be found in it on each side in front of the level 

 of the ventral root of the second nerve. 



In this section is likewise seen a small branch of the ramus lateralis vagi which 

 enters the slit between the supraclavicle and epiotic, descends forwards between these 

 (r' S.), and eventually reaches a neuromast in the supraclavicle. Fig. 13 is the foremost 

 of the sections represented, in which the basioccipital processes of the supraclavicle 

 appear. 



Seven sections intervene between the escape of the second nerve and that of the third 

 (Fig. 14) which, however, has both dorsal and ventral roots developed, both of which 

 escape through the membranous walls of the neural canal. Between the centrum of the 



