812 DIRECTIONS FOB LABORATORY WORK 



XX. THE EYE 



(Bullus Oculi, and Appendages: Palpebrce and Lacrimal Glands) 



(A) MACROSCOPIC STUDY. 



(1) With the aid of a mirror, sketch your eye (374). Note: ar, 

 supercilium (eye-brow) ; b, superior and inferior palpebrae ( eye-lids ), 

 with their cilia (eye-lashes) at the base of which internally may be seen 

 a row of whitish pits, the orifices of the tarsal (Meibomian) glands; c, 

 the medial and lateral anguli. oculi and the canthi (commissures) ; d, in 

 the depth of the medial angle (the lacus lacrimalis) a reddish rough- 

 ened area, the caruncula lacrimalis ; e, at the base of this angle the plica 

 semilunaris (homologue of nictitating membrane or third eye-lid of 

 birds, reptiles, etc.), and f, at either basal angle of the internal canthus 

 an elevated papilla lacrimalis, with its punctum lacrimalis leading to 

 the canaliculus lacrimalis and the naso-lacrimal duct. Note also the 

 fine hairs on the caruncula. Eetract the lower lid and note the con- 

 junctive palpebrae; its continuity with the conjunctiva oculi across the 

 fornix conjunctivas can readily be observed. 



Within the rima palpebrarum, the opening between the lids, note:: 

 a, the pupil; b, the iris; and c, the overlying transparent cornea, con- 

 tinuous peripherally with the opaque sclera. What is the pinguecula? 1 



(2) In a formalin-preserved specimen of the eye-ball of sheep or ox,, 

 dissect out carefully the six extrinsic eye-muscles. Note the point and 

 manner of insertion of the muscles and the relation of the superior and 1 

 inferior obliques to the superior and inferior recti muscles respectively.. 

 The levator palpebrae superioris muscle may also have remained intact; 

 also a muscle attached by a stout tendon to the well-developed carti- 

 laginous plica semilunaris, the musculus plicae. In the eyes of ruminants 

 an additional muscle occurs, not present in the human eye, the retractor: 

 bulbi ; this large stout muscle completely envelops the optic nerve. Sep- 

 arate the retractor bulbi muscle from the optic nerve down to the point: 

 of exit of the nerve from the bulb. By noting the position of the point: 

 of exit with respect to the posterior pole of the bulb, and the relation of 

 this point to the plica semilunaris and levator palpebrae muscle, deter- 

 mine which eye you have been dissecting. Sketch anterior and posterior 

 poles (375). 



(3) Bisect the eye-ball in the horizontal meridian. (This should be 

 done with a sharp razor, proceeding from the posterior pole, first divid- 



