THE DOGFISH 7 



Split the cranium longitudinally in the middle line and study 

 the internal surface. In the floor see how far forward the noto- 

 chord extends. In front of its tip is an internal projection, the 

 clinoid process, in front of which is a depression, the sella 

 turcica, occupied by the hypophysial structures of the brain. 

 Probe the transbasal canal from the outside and see where it 

 appears on the inner surface ; also trace the course of the foramen 

 for the internal carotid artery. Recognize as far as possible the 

 foramina in the orbital wall. Above and a little behind the 

 clinoid process is a double depression for the auditory-facial and 

 Gasserian ganglia. Probe the foramen lacerum and the facial 

 foramen from the outside and see where they connect. Behind 

 the inner end of the facial foramen is the opening for the passage 

 of the auditory nerve into the inner ear. In the same way find 

 the inner end of the vagus foramen and look half-way between 

 vagus and facial for the inner end of the glossopharyngeal foramen. 



Draw the inner surface of the cranium, natural size, indicating 

 the variations in thickness of the walls. 



Cut across the otic region of one-half of the bisected cranium, 

 laying open the otic capsule and its system of cavities, the 

 skeletal labyrinth. See the large central chamber and trace the 

 connection with it of the several semicircular canals, the position 

 of which has been recognized from the surface. 



The visceral skeleton consists of a series of seven visceral 

 arches, two of which are united by ligaments to the cranium, 

 while the others are placed posterior to it. These really belong to 

 the skull, their position being due to growth greater in the ventral 

 region than in the dorsal. This matter of position as well as 

 differences in structure serves to divide the arches into two 

 groups, the posterior five, the branchial or gill arches, being 

 much alike, while the most anterior, the mandibular arch, and 

 the second, or hyoid arch, differ from each other as well as from 

 those farther back. All of the arches present right and left halves 

 and can also be divided into dorsal and ventral portions. 



The first or mandibular arch consists of two cartilages on 

 either side: a dorsal, or pterygoquadrate, which forms the upper 

 jaw, and a ventral, or Meckel's cartilage, the lower jaw. These 

 are hinged at their posterior ends, the angle of the jaws. External 

 to each jaw, and nearly meeting at the angle of the mouth (not 

 jaw), are three labial cartilages on either side. How are they 



