124 THE SKULL [CHAP. 



nerves (the ninth, tenth, and eleventh) pass out of the 

 cranium. 



9. The exoccipital is perforated, a little in front of the 

 condyle, by the condylar foramen, which gives exit to the 

 twelfth, or hypoglossal, nerve. 



10. Lastly, the large median opening, behind the bones 

 of the posterior segment of the skull, is the fora men magnum. 

 through which the spinal cord passes out. 



It will be seen from the foregoing description that the 

 three organs of special sense, situated in the walls of the 

 cranium, have definite relations with the three osseous 

 segments. The first, or organ of smell, is situated in front 

 of the frontal segment ; the second, or organ of sight, 

 receives its nerves by apertures situated between the frontal 

 and parietal segments or perforating the former; the third, 

 or organ of hearing, is intercalated between the parietal 

 and occipital segment. 



The portion of the skull anterior to the junction of the 

 presphenoid and the mesethmoid constitutes the face. This 

 differs entirely from the cranial cavity in having a complete 

 median partition, and consists mainly of two tubular cavities 

 placed one on each side of this partition. These are the 

 nasal cavities. They are open at each end, the orifices 

 being termed respectively anterior and posterior nares. 



Each of these elongated cavities is deepest vertically in 

 its posterior part, where it is partially divided into an upper 

 and lower chamber : the upper one, the olfactory chamber, 

 being closed behind by the cribriform plate ; the lower, the 

 narial passage, terminating in the posterior nares. 



Each nasal cavity may be described as having an inner 

 wall, an outer wall, a floor, and a roof. 



The inner wall is formed mainly by the partially ossified 



