24 



ZOOTOMY. 



88. Two paired, somewhat kidney-shaped masses, forming 

 the anterior division of the brain, and having their convexi- 

 ties towards the middle line ; they are divided by depressions 

 on their outer sides into two parts, the smaller posterior 

 part answering to cerebral hemispheres, or prosence- 

 phala (Fig. 9, c.h), the larger anterior part to olfactory 

 lobes, or rhinencephala {olf^. 



89. The nasal sac (Fig. 4, no), a rounded chamber, 

 opening by a short tubular prolongation in the single nasal 

 aperture. On opening, it is seen to be lined with strongly 

 pigmented epithelium, and to have its walls much plaited. 



Fig. 9. — Petromyzon marinus. The brain from the dorsal aspect, 

 with the right auditory sac (X 3). 



aq.s, aqueduct of Sylvius : a.s.c, anterior semicircular canal : cb, 

 cerebellum: c.h, cerebral hemispheres : m.o, medulla oblongata: o.l, 

 optic lobes : olf^ olfactory lobes ; p.s.c, posterior semicircular canal : th, 

 thalamencephalon : v, vestibule : v.% third ventricle : -'.4, fourth 

 ventricle : I — X, cerebral nerves. 



90. The olfactory nerves (Fig. 9, I), passing directly forwards 

 from the rhinencephala to the nasal sac, through the apertures in the 

 nasal capsule (§ 6). 



91. The optic nerves (Fig. 9, II), springing from the ventral surface 

 of the thalamencephalon and passing directly outwards through the 

 optic foramina (§ 4) to the eyes. 



92. The oculomotor (third) nerves (Fig. 9, III), springing from 



