DISSECTION OF THE SHARK 21 



Very complete descriptions and figures of the ears of different 

 species of fishes, including the dogfish, are given by Retzius 

 ('81). On the functions of the ears of fishes, see the papers by 

 F. S. Lee and G. H. Parker cited in the Bibliography. For the 

 mammalian ear see Section 80. 



10. Remove the cartilaginous walls of the cranium down to 

 the foramina of the cranial nerves. This can best be done by 

 cutting the cartilage away in thin slices with a sharp knife. 

 The foramen of the slender IV nerve will be first exposed, lying 

 medially of the eyeball and far dorsally. The other foramina 

 lie farther ventrally. 



sticp-lateral area 

 Kin area 



Hfuctory bulb 



Optic nerVe- 



Supraorbital trunK 

 Deep ophthalmic neiYe 

 Infraorbital trun 



FIG. 3. The brain of Squalus acanthias seen from left side. Designation of 

 nerve roots and brain centers the same as in Fig. 2. 



11. Note the membranes (meninges) of the brain. The 

 dogfish brain is closely enveloped by a single membrane. Be- 

 tween this and the perichondrium which lines the cartilaginous 

 cranium is a very loose arachnoidal tissue which is not, how- 

 ever, condensed into a definite arachnoid membrane, as in 

 mammals (cf. Section 45). 



12. Carefully dissect out the cranial nerves and eye-muscles of 

 the right side. The optic nerve, eye-muscles, and eye-muscle 

 nerves (III, IV, and VI pairs) are arranged practically as in 

 the human body. Consult your anatomies for names and de- 

 scriptions and determine the action of each of the eye-muscles. 

 Notice the long ciliary nerves distributed to the eyeball and 

 trace their connections, if possible, with branches of the third 

 and fifth nerves. 



