8. There is a complicated system of Eustachian passages communicating at one end with the tympanic cavity and at the other end with the mouth cavity. 9. The interorbital septum is mainly cartilaginous, the presphenoidal and orbitosphenoidal regions remaining unossified. The =skull= is divisible into three parts:-- (1) the cranium, (2) the lower jaw, (3) the hyoid. The =cranium= may again for purposes of description be divided into:-- 1. the cranium proper or brain case; 2. the bones connected with the several special sense organs; 3. the bones of the upper jaw, and suspensorial apparatus. 1. THE CRANIUM PROPER OR BRAIN CASE. [Illustration FIG. 43. PALATAL ASPECT A, OF THE CRANIUM, B, OF THE MANDIBLE OF AN ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.) 1. premaxillae. 2. maxillae. 3. palatine. 4. pterygoid. 5. posterior nares. 6. transpalatine. 7. posterior palatine vacuity. 8. anterior palatine vacuity. 9. basi-occipital. 10. opening of median Eustachian canal. 11. jugal. 12. quadratojugal. 13. quadrate. 14. dentary. 15. splenial. 16. coronoid. 17. supra-angular. 18. angular. 19. articular. 20. lateral temporal fossa. 21. openings of vascular canals leading into alveolar sinus.] The cartilage and membrane bones of the cranium proper when taken together can in most vertebrates be seen to be more or less arranged in three rings or segments called respectively the =occipital=, =parietal= and =frontal= segments; in the Crocodile however only the occipital and parietal segments are clearly seen. The =occipital segment= consists of four cartilage bones, three of which together surround the =foramen magnum=. The most ventral of these, the =basi-occipital= (figs. 43 and 45, 9), forms the single convex =occipital condyle= for articulation with the atlas, bounds the base of the foramen magnum, and is continuous laterally with two larger bones, the =exoccipitals= (fig. 45, 24), which meet one another dorsally and form the remainder of the boundary of the foramen magnum. Each is drawn out externally into a strong process, which is united below with the quadrate, and above with the squamosal by a surface seen in a disarticulated skull to be very rough and splintered. In a longitudinal section the anterior face of the exoccipital is seen to be closely united with the opisthotic. The exoccipital is pierced by a number of foramina, four lying on the posterior surface. Just external to the foramen magnum is a small foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII). External to this is the foramen for the pneumogastric (fig. 44, X), while more ventrally still is the foramen (fig. 44, 15) through which the internal carotid artery enters the skull. Some distance further to the side, and more dorsally, is a larger foramen which gives passage to the facial nerve and certain blood-vessels.