DISSECTION OP THE HEAD AND NECK. 173 



conjunctiva as close-set yellow lines having a direction at right angles to 

 the edge of the lid. They number about fifty or sixty in the upper lid, 

 but they are fewer and less distinct in the lower. Each gland consists 

 of a main tube with lateral sacculi opening into it on each side, and it 

 discharges its secretion by a dot-like orifice on the edge of the eyelid. 

 The free edge of each lid carries a fringe of stiff hairs — the eyelashes, 

 which tend to prevent the entrance of foreign particles into the eyes. 

 The attached edge of each lid is max-ked on the ocular side by the angle 

 of reflection of the conjunctiva from the lid to the eyeball, but on the 

 facial side the eyelid passes into the surrounding skin without any 

 defined line. At each extremity the eyelids join to form a commissure, 

 or canthus. The outer or temjwral canthus is acute, but the inner or 

 nasal canthtis is rounded, and lodges the caruncula lachrymalis. 



The Caruncula Lachrymalis is a small, rounded, and, generally, 

 dark-pigmented nodule placed within the nasal canthus, and about 

 equidistant from the two puncta lachrymalia. It is covered by con- 

 junctiva, and is composed of connective-tissue with some mucous follicles 

 and the bulbs of a few short hairs, which project from it. 



The Membrana Nictitans. This body is placed at the inner canthus, 

 where, ordinarily, it projects to only a slight extent, but it is capable of 

 being thrust more than half way across the front of the eye. It has for 

 its basis a thin and flexible piece of elastic cartilage, which anteriorly is 

 invested by conjunctiva. Posteriorly this cartilage passes to the inner 

 side of the eyeball, where it becomes connected with the cushion of 

 semifluid fat which is found in the posterior part of the orbit. The 

 membrana nictitans has no muscle to move it directly; but when the 

 eyeball is retracted within the orbit, it presses on the semifluid fat 

 behind it, and this, tending to escape at the side of the eyeball, pushes 

 the membrana nictitans before it. In the eye of a subject just dead, 

 this mechanism may readily be demonstrated by pressing the eyeball 

 backwards into the orbit. About the centre of the outer face of the 



cartilage, there will be found a cluster of reddish-yellow granules the 



Harderian gland. The gland secretes an unctuous material which is 

 discharged by a number of ducts that perforate the cartilage and open 

 on its ocular surface. 



The Lachrymal ApjMratus comprises — the lachrymal gland with its 

 excretory ducts, the puncta lachrymalia, the lachrymal canals, the 

 lachrymal sac, and the lachrymal duct. 



The Lachrymal Gland is placed within the orbit, beneath the supra- 

 orbital process of the frontal bone. The gland itself will be dissected at 

 a later stage. 



The excretory ducts of the gland discharge themselves by a number 

 of minute openings on the inner surface of the upper eyelid, close to the 

 temporal canthus. Sometimes a few of the ducts open on the lower lid 



