76 THE VERTEBRATE SKELETON. 



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 Vila.), 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). 



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 post- 

 orbital 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 pneumo- 

 gastric foramina for the passage of the tenth nerves, and 

 further to the sides, just beyond the posterior vertical semi- 

 circular 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 inter-nasal septum and terminated 

 laterally by the sub-orbital 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. 



