144^ 



HUMAN ANATOMY. 



FIG. 1200. 



of its ends, the angles or canthi, lies slightly (from 4-6 mm. ) lower than the other one. 

 The free borders of the lids meet at the outer canthus without change of curvature, 

 but on approaching the inner canthus they alter their direction and extend medially 

 for several millimeters before meeting. In this manner immediately external to 



the inner canthus the lids bound a shallow 

 D-shaped recess, about 5 mm. long, known 

 as the lachrymal lake < lacus lacrima 



The palpebral fissure, which possesses an 

 average length of 30 mm. and a height of from 

 12-14 mm. , is subject to considerable individ- 

 ual variation in size, thereby exposing a vari- 

 able amount of the eyeball. In consequence, 

 the appearance of a larger or smaller eye 

 is produced, an impression, however, that 

 depends upon the size of the opening between 

 the lids and not upon differences in the eyeball 

 itself, the diameters of which, under normal 

 conditions, are practically constant. The 

 height of the palpebral fissure in young 

 children is relatively greater than in the adult, 

 a peculiarity that confers the characteristic 

 wide-eyed appearance in early life. 



The upper lid is not only much the 

 broader, its height being about double that 

 of the lower one, but also the more movable 

 and the chief agent in closing the palpebral 

 opening. When the latter is closed the free 

 edges of the two lids are in contact through- 

 out their length, the anterior margin of the 

 upper one overlapping slightly the corre- 

 sponding edge of the lower. The line of 

 apposition is somewhat arched, with the 

 convexity directed downward, and falls below 

 a horizontal line passing through the inner 

 canthus. When the eyelids are separated 

 to the usual extent, the free edge of the 

 upper lid lies just below the upper margin 

 of the cornea, a narrow cresoentic area of 

 which it masks, while the corresponding 

 border of the lower lid falls slightly I 

 the inferior corneal margin. The position 

 of the pupil is about midway between the 

 two canthi. When the eyelids are closed, 

 the upper fold covers the entire cornea, its 

 border lying opposite the correspond- 

 ing margin of the cor: 



ed in s,\^ ,. 1 201), 



the free border of the lid presents a well- 

 defined posterior margin, along which open 

 the minute ducts of the tarsal glands, 



whilst the anterior margin is rounded and passes insensibly into the adjoining 

 external skin-surface and is tvset with the eyelashes. The latter, the cilia, ar 

 outwardly curving hairs, which number from 100-150 in the upper lid and about half 

 as many in the lower. With the exception of about 5 mm. next the inner angle, 

 where the lids border the lachrymal lake and the ev . the cili 



arranged in a double or triple row. with the loiu; in the centre of the 



upper series. Although their follicles occupy a zone of from 1-2 mm. in width, the 

 free ends of the cilia 1:< illy in a single row. the I : more C 



upper lashes either crossing or overlying the shorter ones of the lower 





Three vie\v ve. showing relations of 



eyeball to palpebral fissure and details of inner 

 canthus. 





