the skull; through it the ophthalmic branches of the fifth and seventh nerves pass. Slightly further back near the ventral surface is the large optic foramen (fig. 6, II.) for the passage of the second nerve. Vertically above the optic foramen, near the dorsal surface, is the very small foramen for the fourth nerve (fig. 6, IV.). Behind and a little above the optic foramen is another small aperture, the foramen for the third nerve. Behind and slightly below this is the large foramen for the sixth and main branches of the fifth and seventh nerves (fig. 6, V.). In front of and slightly below this foramen are seen two other small apertures; the more anterior and ventral of these (fig. 6, 4) is for the passage of a vessel connecting the efferent artery of the hyoid gill with the internal carotid artery inside the skull, the more posterior and dorsal is for the interorbital canal (fig. 6, 3) which unites the two orbital sinuses. Above and very slightly in front of the large foramen for the sixth and main parts of the fifth and seventh nerves, are two small foramina (fig. 6, Va., and VIIa.), through which the ophthalmic branches of the fifth and seventh nerves enter the orbit. Behind and slightly below the large foramen just mentioned is a small hole through which the external carotid enters the orbit (fig. 6, 9).
| 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.
The jaws are developed from a structure whose dorsal and ventral portions subsequently become of very different importance. The ventral portion forms both upper and lower jaws, the former being developed as an outgrowth from the latter. The dorsal portion forms only the pre-spiracular ligament (fig. 6, 20), a strong fibrous band containing a nodule of cartilage, and running from the anterior part of the auditory capsule to the point where the jaws are connected with the hyomandibular.
(b) The hyoid arch consists of a pair of cartilaginous rods which are attached at their dorsal ends to the cranium, and are united ventrally by a broad median plate of cartilage, the basi-hyal. Each rod is divided into a dorsal portion, the hyomandibular and a ventral portion, the cerato-hyal. The hyomandibular (fig. 6, 13) is a short stout rod of cartilage projecting outwards, and somewhat backwards and downwards from the cranium, with which it articulates behind the orbit and below the postorbital groove. Its distal end articulates with a rather long slender bar, the cerato-hyal (fig. 6, 14), which is in its turn attached to the side of the basi-hyal. The basi-hyal is a broad plate, rounded in front and drawn out behind into two processes to which the two halves of the first branchial arch are attached. The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, the gill rays. The hyoid arch forms the main suspensorium or means by which the jaws are attached to the cranium. This attachment is chiefly brought about by a series of short ligaments which connect the posterior ends of both upper and lower jaws with the hyomandibular, but there is also a ligament connecting the lower jaw with the cerato-hyal. The attachment of the jaws to the cranium is also partially effected by the pre-spiracular and ethmo-palatine ligaments.
(c) Each of the five branchial arches is a hoop, incomplete above and formed of four or more pieces of cartilage. The most dorsal elements, the pharyngo-branchials, are flattened, pointed plates whose free inner