208 THE DOG-FISH 



end, just in front of the auditory capsule, and 

 above the foramen for the main branches of 

 the fifth and seventh nerves : the aperture for 

 the ophthalmic branch of the seventh nerve 

 is the larger of the two, and placed slightly 

 above and behind that for the fifth. From 

 these apertures shallow grooves run forwards 

 along the inner wall of the orbit, close to its 

 dorsal border, to the foramen by which the 

 two nerves leave the orbit : this is situated 

 at the antero-superior angle of the orbit, and 

 has already been seen on the dorsal surface 

 of the skull. 



c. The articular surface for the hyo-mandibular car- 



tilage is a concave depression on the side of the 

 hinder end of the skull, below the auditory cap- 

 sule. 



d. The post-orbital groove is a deep horizontal groove 



immediately above the articular surface for the 

 hyo-mandibular cartilage, and overhung by the 

 projecting ledge of the auditory capsule which 

 lodges the horizontal semicircular canal : it lodges 

 during life the venous canal through which the 

 orbital sinus communicates with the anterior 

 cardinal sinus. 



The anterior end of the groove is bridged over 

 by the upper end pi the stout pre-spiracular liga- 

 ment. 



e. The foramen for the glosso-pharyngeal nerve is at 



the hinder end of the floor of the post-orbital 

 groove, behind and below the horizontal semi- 

 circular canal of the ear. 



4. The ventral surface of the skull. 



a. The under surface of the skull is a broad flat plate 

 of cartilage, crossed almost transversely a little 

 behind its middle by a pair of grooves in which 

 the carotid arteries lie. At the point of meeting 



