40 A TYPICAL VERTEBRATE EYE: THE HUMAN 



therefrom in the embryo and sometimes bears eyelashes and their assoc- 

 iated glands as evidence of its true nature. 



Near the inner canthus on each Ud margin is a pore raised on an 

 eminence. These 'punctae lacrimaha' are exits for the tear fluid which 

 accumulates in a pool, the lacus lacrimalis, at the inner canthus. 



The human upper lid (Fig. 18) does most of the work in closing the 

 eye, though in most vertebrates it is the lower which moves the more. 

 A continuous sphincter muscle surrounds the palpebral fissure and is 

 much flattened and very broad where it courses through the two lids 

 between their outer dermal and inner conjunctival surfaces. The oppo- 

 nents of this 'orbicularis oculi' muscle are thin muscles running down 

 into the upper lid and up into the lower. The more important of these 

 is the levator muscle of the upper lid, which works with the superior 

 rectus of which it is a derivative. Thus, when the eyeball is turned up- 

 ward the lid automatically rises. When the levator is paralyzed, as 

 sometimes occurs in diseases of the nervous system, the individual has 

 a sleepy look owing to the unsightly drooping of the lid; but the oph- 

 thalmic surgeon cleverly corrects this by fastening the inside of the lid 

 to the superior rectus itself. 



Between the muscle-sheets of the lids and their conjunctival linings 

 lie firm plates, one in each lid — the tarsi. Each tarsus is composed of 

 dense connective tissue and is curved to fit the surface of the eyeball. 

 Their presence insures a smooth sliding of the lids and obviates any 

 tendency of the latter to roll up when in action. Embedded in each 

 tarsus is a row of elongated (Meibomian) glands which open by a series 

 of apertures behind the lid margin. They represent an additional row of 

 eyelashes which have disappeared in evolution, leaving their glands 

 behind them. The sebaceous secretion of these, together with that of 

 smaller glands (of Zeis) associated with the lashes which are scattered 

 along the edges of the lids, maintains a film of oily emulsion over the 

 layer of tear fluid and holds the latter firmly and smoothly against the 

 eyeball. The tears can spill over onto the cheeks only when they so 

 accumulate that their weight breaks the retaining film. 



The periodic blinking of the lids is ordinarily involuntary and un- 

 conscious. The rate of blinking varies, but each blink occupies %o of a 

 second. Its chief values are in moistening and cleaning the cornea and 

 in pumping the tear fluid out of the lacus lacrimalis — though this is an 

 incidental function of the lid muscles rather than of the lids themselves. 

 One might expect the drying of the cornea to initiate the blinking reflex, 



