[a]Fig. 6. Lateral view of the skull of a Dogfish] (Scyllium
canicula) × 2/3.
| 1. nasal capsule. | 10. ethmo-palatine ligament. |
| 2. rostrum. | 11. palato-pterygo-quadrate bar. |
| 3. interorbital canal. | 12. Meckel's cartilage. |
| 4. foramen for hyoidean artery. | 13. hyomandibular. |
| 5. foramen for the exit of the | 14. cerato-hyal. |
| ophthalmic branches of | 15. pharyngo-branchial. |
| Vth and VIIth nerves. | 16. epi-branchial. |
| 6. foramen through which the | 17. cerato-branchial. |
| external carotid leaves the | 18. gill filaments, nearly all have |
| orbit. | been cut off short for the |
| 7. orbitonasal foramen. | sake of clearness. |
| 8. auditory capsule. | 19. extra-branchial |
| 9. foramen through which the | 20. pre-spiracular ligament. |
| external carotid enters the | II. III. IV. V. Va. VIIa. foramina |
| orbit. | for passage of cranial nerves. |
Behind the orbit is the auditory capsule. This is marked below by a prominent surface for the articulation of the hyomandibular, above which is the deep postorbital groove for the passage of a blood-vessel, connecting the orbital and anterior cardinal sinuses.
(c) Passing to the posterior end of the cranium: in the centre is seen the large foramen magnum through which the brain and spinal cord communicate. The notochord enters the skull just below this foramen, and on each side of the notochord is a projection, the occipital condyle, by which the first vertebra articulates with the skull.
External to the condyles are the prominent pneumogastric foramina for the passage of the tenth nerves, and further to the sides, just beyond the posterior vertical semicircular canals, are a pair of deep pits in which lie the foramina for the ninth nerves (fig. 6, IX).
(d) The broad and flat ventral surface of the cranium is continued in front as the internasal septum and terminated laterally by the suborbital ridges. At a little behind the middle it is traversed by two shallow grooves along which the internal carotid arteries run. At the divergent ends of these grooves are seen two small apertures through which the external carotids enter the orbit (fig. 6, 9), and at the point where they meet is a single small aperture through which the internal carotid enters the cranium.
(2) The Visceral Skeleton.
The Visceral skeleton forms a series of seven cartilaginous arches or hoops, surrounding the anterior part of the alimentary canal, and enclosing a wide but rather shallow space.
(a) The first or mandibular arch is the largest of the series, and forms the upper and lower jaws. Each half of the upper jaw or palato-pterygo-quadrate bar is formed by a thick cartilaginous rod which meets its fellow in the middle line in front, the two being united by ligament. Each half is connected to the cranium just in front of the orbit by the ethmo-palatine ligament (fig. 6, 10), and at its hind end articulates with one of the halves of the lower jaw. Each half of the lower jaw or Meckel's cartilage (fig. 6, 12) is a cartilaginous bar, wide behind but narrow in front, where it is united to its fellow by a median ligament. Imbedded in the tissue external to the upper jaw are a pair of labial cartilages, and a similar but smaller pair are imbedded in the tissue external to the lower jaw.