THE DOGFISH 5 



The skull consists of a cranium which encloses the brain and 

 the special sense-organs, and of a visceral skeleton, developed 

 in the walls of the mouth and pharynx, which is more loosely 

 connected to the cranium in the Elasmobranchs than in most 

 vertebrates, hence especially interesting as explaining structures 

 in the other groups. 



In the cranium do you find joints (sutures) separating it into 

 parts such as are noticeable in the skulls of other vertebrates? 

 Make out in it a median portion, the brain-case, with at either 

 side, in front, a nasal capsule, and a projection behind, the 

 otic capsule, the space between the two being the orbit. The 

 anterior median prolongations of the brain-case in front of the 

 nasal capsule is the rostrum. In Mustelus and some other sharks 

 this consists of three bars, one median and ventral the others 

 dorsal and paired. In Acanthias the three are united by car- 

 tilage into a single structure. Are there any large gaps (fonta- 

 nelles) in the floor or roof of the brain-case? 



In the base of the cranium (i.e., that side of it which joins 

 the trunk) is a large opening, the foramen magnum, for the 

 passage of the spinal cord. Ventral to this is a cup-like depres- 

 sion (posterior end of cranial notochord) similar to those in the 

 vertebral centra. Lateral to this are a pair of small projections 

 (occipital condyles); still more lateral, a large foramen for the 

 vagus nerve, and nearly at the postero-lateral angle of the skull 

 a similar foramen for the glossopharyngeal nerve. 



Draw the base of the cranium natural size, including the otic 

 capsules, naming parts. 



In the dorsal view the foramen magnum is still visible. A 

 little in front of it is a large pit with the paired openings of the 

 endolymph-ducts in its bottom. Lateral to this pit and extend- 

 ng backward to the base of the cranium and forward to the 

 strong postorbital process is the otic capsule. The orbits are, 

 in part, overhung by a strong supraorbital crest, extending from 

 the postorbital to a preorbital process, which bounds the nasal 

 capsule behind. Note the row of small foramina (for twigs of the 

 superficial ophthalmic nerve) at the base of the crest, and, farther 

 forward, a longer ophthalmic foramen by which this nerve 

 emerges to the top of the skull. In front of the ophthalmic 

 foramen are two ethmoidal canals. Probe them and see where 

 they lead. Between the roof (tegmen cranii) of the brain-case 



