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.ti), the larger anterior part to olfactory 

 lobes, or rhinencephala (olf). 



89. The nasal sac (Fig. 4, na\ 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. 



? . S .C 



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 : ///, 

 thalamencephalon : v, vestibule : ^.3, third ventricle : i>.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 



