1 84 VERTEBRATE ZOOLOGY 



elongated structures; the third lies behind and somewhat above 

 the middle turbinal, at the hinder end of the nasal cavity. It is 

 much smaller than the others and the only one to which the ol- 

 factory nerve goes. 



Exercise 25. Draw the nasal cavity. 



The Eye. Note the large size of the eye. Observe the front 

 aspect of it. The outer layer is the cornea, which is transparent. 

 It is a continuation of the black sclerotic, which is the outer layer 

 of the back part of the eye; note the bony ring in this layer. 

 Through the transparent cornea the iris and pupil will be seen. 

 The former is a yellow ring; the latter, the round opening into 

 the interior of the eye. Lining the inner surface of the eyelids and 

 forming the outer surface of the cornea is a transparent membrane 

 called the conjunctiva. 



Exercise 26. Draw these features. 



The Muscles of the Eyeball. Cut away the bony ridge which 

 surrounds the right eye and thoroughly expose the orbit, but with- 

 out disturbing the eyeball or its muscles, or cutting through the 

 thin skull into the brain. By pressing the eyeball backward, the 

 insertions of two muscles, the superior oblique and the inferior 

 oblique, are brought into view, that of the former being on the 

 medial surface of the eye and that of the latter on the antero- 

 ventral surface. Both muscles have their origins on the anterior 

 wall of the orbit. Beneath the inferior oblique and between it and 

 the wall of the orbit will be found the Harderian lachrymal gland, 

 a small, fatlike body which moistens the nictitating membrane. 

 The lachrymal gland is a very small, white body at the hinder side 

 of the eye and may not be seen. 



Just back of the insertion of the superior oblique is that of the 

 superior rectus muscle. Pull the eye forward ; the insertion of the 

 external (posterior) rectus will be seen on the posterior side of 

 the eyeball. Cut the superior oblique at its insertion and pull the 

 eye backward ; beneath it will be seen the insertion of the inter- 

 nal (anterior) rectus muscle. Cut all these muscles at their in- 

 sertions and pull the eyeball backward ; the inferior rectus, which 



