84 PRACTICAL ZOOLOGY. 



away completely the lower jaw and the floor of the 

 mouth. Move the gill-covers in and out to show 

 more clearly the thin plates of cartilage between the 

 eyes and the roof of the mouth ; with scissors slit in 

 the middle line the tough membrane lining the roof of 

 the mouth, and strip it out to the sides. Observe a 

 muscle running outward from each side of the base of 

 the skull to the corresponding gill-cover. Cut these 

 at their inner ends and turn them outward. With 

 scissors cut away the cartilages covering the under 

 surfaces of the eyes. 



3. Observe a muscle passing outward from the front part 

 of the socket to the eyeball, the inferior oblique 

 muscle. 



4. The muscle running forward close to the partition 

 between the eyes is the internal rectus. 



5. On the under surface of the eye is the inferior rectus. 



6. Attached to the hinder border of the eye is the larger 

 external rectus. Note carefully the origin of each 

 of these, their place of insertion on the eyeball, and 

 their change of shape in their course ; consider the 

 effect of each on the eye. 



Observe the thin-walled swellings at the sides of 

 the base of the hinder part of the skull; cut into 

 these ear-capsules and find in each a membranous 

 sac, the vestibule of the ear. In this sac lies the 

 " ear-bone" or otolith. Find the white optic nerve 

 arising from the inner surface of the eyeball; with 

 a sharp knife cautiously cut away the base of the 

 skull and trace the optic nerves to the brain ; de- 

 monstrate that they cross each other, the optic 

 nerve from the right eye entering the left half of 

 the brain, and vice versa. 



