28 VERTEBRATE ZOOLOGY 



skeleton, which incloses the anterior end of the alimentary canal 

 and forms the gill arches and the skeleton of the mouth. Separate 

 the cranium from the visceral skeleton and thoroughly clean 

 the former. 



The cranium is an irregularly shaped case of cartilage. The 

 orbit, which contains the eye and its muscles and nerves, is a 

 long, deep depression occupying the larger part of the side of it ; 

 behind this on each side is the auditory capsule, which thus forms 

 the hinder end of the cranium ; at the forward end of the orbit 

 is the nasal capsule. The rostrum, the framework of the snout, 

 forms the anterior portion of the skull. In the smooth dogfish it is 

 formed of three delicate rods of cartilage which meet at their for- 

 ward end; in the spiny dogfish the spaces between the rods are 

 filled with thin plates of cartilage, giving the rostrum a troughlike 

 form, open dorsally. Just behind the nasal capsule on each side of 

 the dorsal surface is the large ophthalmic foramen, through which 

 the superficial ophthalmic branches of the trigeminus and facial 

 nerves find their exit from the orbit. In the dorsal surface of the 

 hinder part of the cranium is a deep depression, at the bottom of 

 which are two small holes ; these are the openings of the endolym- 

 phatic ducts. 



At the hinder end of the cranium is the large foramen magnum, 

 through which the spinal cord enters the brain cavity. Beneath it 

 are the surfaces by which the skull articulates with the backbone, 

 — in the median area a deep depression which joins the centrum 

 of the first vertebra, and on each side of it a slender projection, 

 the occipital condyle. Lateral to each condyle is the large vagus 

 foramen, for the exit of the vagus nerve from the brain case. The 

 glossopharyngeal foramen for the passage of the glossopharyngeal 

 nerve is in the posterolateral angle of the cranium. 



The ventral surface of the cranium is broad and flat posteriorly, 

 where it forms the roof of the mouth, and narrow between the 

 orbits. A median ridge, or keel, runs the length of it, through which 

 will be seen, between the auditory capsules, the notochord, which 

 thus extends forward as far as the hinder end of the orbits ; at its 

 anterior end is a small foramen for the passage of the internal 

 carotid artery. At the base of the rostrum are two large openings 



