98 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the chondrocranium presents a large depression, the orbit, which in life holds 

 the eye. 



a) Dorsal surface of the chondrocranium: Study the dorsal surface. The 

 rostrum opens dorsally by a large, egg-shaped cavity, the anterior fontanelle. 

 At either side of the base of the rostrum is an olfactory capsule, a projecting 

 structure with thin walls. Posterior to the capsule and continuous with its 

 dorsal wall is a thick projecting shelf, the supraorbital crest, which forms the 

 dorsal wall of the orbit. The posterior end of the crest continues into a pro- 

 jection, the postorbital process. Along the medial borders of the supraorbital 

 crests runs a row of openings, which are nerve foramina, i.e., openings through 

 which nerves pass to and from the brain. In the median line just back of the 

 anterior fontanelle is an opening, the epiphyseal foramen, through which a por- 

 tion of the brain (epiphysis or pineal body) extends. In the median line of the 

 posterior part of the roof is a rounded depression, the endolymphatic fossa, in which 

 are situated two pairs of openings, the terminations of the endolymph (smaller 

 outer holes) and perilymph (larger medial holes) ducts of the internal ear. These 

 ducts are canals which connect the fluid-filled channels of the ear with the surface 

 of the skull. On each side of the endolymphatic fossa is a massive region which 

 is the auditory or otic capsule, fused to the cranium. The posterior end of the 

 cranium bears an opening, the foramen magnum, visible dorsally just back of 

 the endolymphatic fossa. Through this opening the brain is continuous with 

 the spinal cord. Draw the chondrocranium from the dorsal side. 



b) Ventral and lateral surfaces of the chondrocranium: The ventral surface of 

 the rostrum bears a projecting carina or keel. On either side of this medial keel 

 is an oval opening, which may be designated the rostral fenestra. Through the 

 rostral fenestrae one can look into the cavity of the cranium, which in life is 

 occupied by the brain. Lateral to each rostral fenestra is the thin-walled, 

 olfactory capsule. In most specimens part of the walls of this will probably 

 be broken away. When complete, the nasal capsule is a nearly spherical struc- 

 ture with a relatively small opening to the exterior. At the bottom of the nasal 

 capsule is a large opening which leads into the cavity of the skull. Posterior 

 to each nasal capsule and continuous with its posterior wall is the preorbital 

 process, which forms the anterior wall of the orbit. The walls of the orbit are 

 pierced by nerve foramina of which the largest, located in the anteroventral 

 region of the orbit, is the optic foramen, through which the optic nerves pass 

 from the eye. 1 The ventral wall of the skull between the two orbits is rather 

 narrow, its narrowest portion being the place where a process of the upper jaw 

 articulates. The posterior part of the ventral surface forms a broad basal plate, 

 whose sides are composed of the otic capsules. In the median line of the basal 

 plate will be seen a streak of slightly different color from the chondrocranium. 



1 Projecting into the orbit may be present a mushroom-shaped structure, the optic pedicel, which 

 in life supports the eyeball. 



