8o Practical Zoology. 



2. Another muscle coming from the posterior part of the socket 

 will be seen passing forward to be attached under the oblique 

 muscle. This is the superior rectus. Make a drawing showing 

 these muscles. The other eye muscles may be more easily ex- 

 amined from beneath. 



If the under surface of the skull of the specimen previously 

 studied be not injured, it may be used ; otherwise, cut off the 

 head of another fish, and cut 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 ex- 

 ternal 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 cau- 

 tiously cut away the base of the skull and trace the optic nerves to the 

 brain ; demonstrate that they cross each other, the optic nerve from 

 the right eye entering the left half of the brain, and vice versa. 



Make a drawing showing this view of the brain and eyes ; oj 

 one of the eyes and remove the spherical crystalline lens. 



