302 VERTEBRATE ZOOLOGY 



the pigmented iris to be seen through it ; in the middle of the iris 

 is the pupil, the opening through which light is admitted into the 

 interior of the eye and which can be contracted or enlarged by 

 the action of delicate muscle fibers. 



Exercise 27. Draw a sketch of the exposed portion of the eyeball to- 

 gether with the lids. 



Carefully cut away the eyelids and the bony arch which forms 

 the dorsolateral wall of the orbit. Observe the lachrymal gland ; 

 it is a large, flattened, reddish body, which lies close to the post- 

 dorsal angle of the eye, in contact with the bony arch, and com- 

 municates with the inner surface of the upper eyelid by several 

 delicate ducts. The fluid furnished by the gland bathes the cornea 

 and is then drained off through two minute lachrymal ducts which 

 open at the edge of the upper and lower lids near the inner angle 

 of the eye. These two ducts join to form the nasolachrymal duct, 

 which passes through the bony wall of the orbit and opens into 

 the nasal cavity. 



Two additional glands lie in the anterior portion of the orbit, 

 — the Harderian and the infra-orbital gland. The Harderian gland 

 is a large, compact body which opens on the inner surface of the 

 nictitating membrane, and consists of two portions,— a lateral, 

 pinkish portion and a medial, white portion; the infra-orbital 

 gland is a diffuse, yellowish salivary gland which lies along the 

 antero-ventral surface of the eyeball, with a duct which passes to 

 the inner surface of the cheek. 



The Muscles of the Orbit. Ten small muscles control the move- 

 ments of the eyeball. Eight of these are muscles which pass 

 straight from the inner portion of the orbit, near the entrance of 

 the optic nerve, to the sides of the eyeball; the four larger are 

 the rectus muscles, and the four smaller, which surround the optic 

 nerve,— the four heads forming a single muscle,— are the retractor 

 muscle. The other two muscles are the inferior and superior ob- 

 lique muscles which have an oblique position. 



The lateral and posterior portions of the bony orbit being re- 

 moved and the glands above mentioned identified, note the muscle 

 which is the most anterior in position of those exposed : it lies on 



