1020 ORGANS OF SPECIAL SENSE 



parallel course, and are here devoid of lashes. Through the open palpebral aperture 

 the front of the eyeball comes into view, extending quite to the outer, but not 

 reaching as far as the inner, canthus; just within the latter we find a small reddish 

 prominence, the lachrymal caruncle; and between this and the eyeball a fold ot 

 conjunctiva known as the plica semilunaris. While the eye is open, press one 

 finger on the skin, a little beyond the outer oanthus, and draw it firmly outwards 

 from the middle line; observe that the upper lid then falls over the eyeball, and 

 that the outline of a firm band already referred to (the tendo oculi) becomes evident, 

 passing between the inner canthus and the nose. The falling of the lid is caus< .1 

 by our dragging upon a ligament (the outer palpebral raphe) to which the outer 

 end of its tarsus is attached, and so putting the lid itself upon the stretch. If, 

 while the eyeball is directed downwards, we place one finger on the outer end of the 

 upper eyelid and draw it forcibly upwards and outwards, we can usually cause the 

 lower division of the lachrymal "land to present just above the outer canthus. 



The upper eyelid is much broader than the lower, extending upwards as far as 

 the eyebrow. The skin covering it is loosely attached to the subjacent tissues above, 

 but more firmly below, nearer the free margin, where it overlies a firm fibrous tis- 

 sue called the tarsus. When the eye is open, a fold is present at the upper border 

 of this latter more tightly applied portion of skin, called the superior palpebral 

 fold, and by it the lid is marked off into an upper or orbital, and a lower or tarsal, 

 division. The presence of the tarsus can be readily appreciated on our pinching 

 horizontally the entire thickness of the eyelid below the palpebral fold. The lower 

 eyelid is similarly divided anatomically into a tarsal and an orbital part, but the 

 demarcation is sometimes unrecognisable on the surface, though there is usually 

 here also a fold or groove (the inferior palpebral) visible when the eye is widely 

 opened. There is no precise limit of this lid below, but it may be regarded as 

 extending to the level of the lower margin of the orbit. Numerous very fine short 

 hairs are seen on the cutaneous surface of both eyelids. The free margin of each 

 lid has two edges (a) An outer, or anterior, rounded edge, along which the stiff 

 eyelashes, or cilia, are closely placed in several rows; and (b) a sharp posterior 

 edge, which is applied to the surface of the globe (see fig. 743). The lashes of both 

 eyelids have their points turned away from the palpebral aperture, so that the upper 

 ones curve upwards, and the lower downwards; the cilia of the upper lid are the 

 stronger, and those in the middle of each row are longer than those at each end. 

 Between the two edges just described, the lid-margin has a smooth surface, on which 

 we observe a single row of minute apertures, which are the openings of large modi- 

 fied sebaceous glands (the tarsal or Meibomian glands) ; it is by these glistening, 

 well-lubricated surfaces that the opposite lids come into apposition when they are 

 closed. The sharp posterior edge of the lid-margin marks the situation of the transi- 

 tion of skin into mucous membrane. Not far from the inner end of this edge we 

 find a prominence, the lachrymal papilla, on the summit of which is a small hole 

 (lachrymal punctum), the opening of the canaliculus for the passage of tears into 

 the lachrymal sac. The lower punctum is rather larger than the upper, and is 

 placed further from the inner canthus. 



If we now examine the inner surface of the eyelids e. g., of the lower we 

 observe that it is lined by a soft mucous membrane, the palpebral conjunctiva. 

 Over the tarsal part of the lid the conjunctiva is closely adherent, but beyond this 

 it is freely movable along with the loose submucous tissue here present. On tracing 

 it backwards, we find that it covers the whole inner surface of the lids, and is then 

 continued forwards over the front of the eyeball, forming the ocular conjunctiva; 

 the bend it makes as it changes its direction here is called the conjunctival cul-de-sac, 

 or fornix. Numerous underlying blood-vessels are visible through the palpebral 

 conjunctiva, and beneath the tarsal part of it we can see a series of nearly straight, 

 parallel, light yellow lines, arranged perpendicularly to the free margin of the 

 lid the tarsal glands. The conjunctiva over the outer and inner fourths of each 

 lid is not quite so smooth as elsewhere, and is normally of a deeper red colour; we 

 shall find later that there are glands well developed in these positions. 



When the eyelids are opened naturally, we see through the palpebral aperture 

 the following: the greater part of the transparent cornea, and behind it the coloured 

 iris with the pupil in its centre; white sclerotic to the outer and inner sides 'of the 

 cornea; the semilunar fold and lachrymal caruncle at the inner canthus. The 

 extent of the eyeball visible in this way varies according to its position. Thus, 



