74- . THE VERTEBRATE SKELETON. 



to the olfactory capsules, and behind owing to the auditory 

 capsules, while in the middle they are deeply hollowed to form 

 the orbits. 



(a) On the dorsal surface of the cranium the following 

 points should be noticed. First at the anterior end, the large 

 thin-walled nasal or olfactory capsules (fig. 6, 1), each of 

 which is drawn out into a narrow cartilaginous process. 



The olfactory capsules have no ventral walls, and are 

 separated from one another by the internasal septum, which 

 is drawn out into a third slender process. These three pro- 

 cesses together constitute the rostrum (fig. 6, 2). 



Behind the olfactory capsules comes a large, nearly circular, 

 hole, the anterior fontanelle, slightly behind which are the 

 two ophthalmic foramina. The dorsal and ventral bound- 

 aries of the orbits are respectively formed by the prominent 

 supra-orbital and sub-orbital ridges. Behind are the audi- 

 tory capsules (fig. 6, 8), each of which is marked by a pair 

 of prominent ridges, converging towards the middle line to a 

 pair of apertures. These apertures communicate with two 

 canals, the aqueductus vestibuli, which lead into the in- 

 ternal ear. The two ridges lodge respectively the anterior 

 and posterior vertical semicircular canals of the ear. 



(6) The principal structures to be noted in a side view of 

 the cranium are contained in the orbit or eye-cavity. Near the 

 base of the orbit at its anterior end is seen the small orbito- 

 nasal foramen (fig. 6, 7), for the passage of blood-vessels, 

 not nerves. Above it is the large ophthalmic foramen 

 (fig. 6, 5) so prominent in a dorsal view of 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 



