1054 SPECIAL SENSE ORGANS 



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 sm-face, on which is a single row of minute apertures, 

 the openings of large modified 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 secretion of these glands is known as the sebum paipebrale. The sharp posterior 

 edge of the lid-margin marks the situation of the transition of skin into mucous membrane. 

 Near the medial end of the margin of the lids we find a prominence, the lacrimal papilla, on the 

 summit of which is a small hole [punctum lacrimale], the opening of the lacrimal duct (ductus 

 lacrimalis) for the passage of tears into the lacrimal sac. The lower punctum is rather larger 

 than the upper, and is placed further from the medial angle of the eye. 



If we now examine the posterior surface of the eyelids — e. g., of the lower — we observe 

 that it is lined by a soft mucous membrane, the palpebral conjunctiva [tunica conjunctiva 

 palpebrarum]. Over the tarsal part of the lid the conjunctiva is closely adherent, but beyond 

 this it is freeh^ movable along with the loose submucous tissue here present. On tracing it 

 backward, we find that it covers the whole posterior sui'face of the lids, and is then continued 

 forward over the front of the eyeball, forming the conjunctival tunic of the globe [tunica con- 

 junctiva bulbi]. The bend it makes as it changes its direction here is called the conjunctival 

 fornix [fornix conjunctivae superior or inferior]. Numerous underlying blood-vessels are visible 

 through the palpebral conjunctiva, and under cover of its tarsal part 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 medial and lateral 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 sclera to the medial and lateral sides of the cornea; the semilunar fold and lacrimal 

 caruncle at the medial angle. The extent of the eyeball visible in this way varies according to 

 its position. Thus, with the eyes looking straight forward, the lower margin of the upper lid 

 is nearly opposite to the top of the cornea, or, more strictly, to a line midway between the top 

 of the cornea and the upper border of the pupil, while the lower lid corresponds with the lower 

 margin of the cornea. When the eyes are directed strongly upward, the upper lid is relatively 

 on a slightly higher level, as it is simultaneously raised, but the lower lid now leaves a strip of 

 sclera exposed Taelow the cornea. On looking downward the upper lid covers the upper part 

 of the cornea as low down as the level of the top of the pupil, while the lower lid is about mid- 

 way between the pupil and the lower margin of the cornea. 



If we draw the eyelids forcibly apart, we expose the whole cornea, and a zone of sclera 

 about eight and a half millimetres in breadth above and below, and ten millimetres in breadth 

 to the lateral and medial sides — altof^ether about one-third of the globe; all the eyeball thus 

 exposed is covered by the ocular conjunctiva [tunica conjunctiva bulbi]. Over the sclera the 

 conjunctiva is freely movable, and through it we see superficial blood-vessels that can be made 

 to slip from side to side along with it (episcleral vessels). Occasionally other deeper vessels 

 may also be seen which do not move with the conjunctiva, but are attached to the sclera (an- 

 terior ciliary arteries and veins). Near the corneal border the conjunctiva ceases to be fieely 

 movable, and it is closely adherent to the whole anterior surface of the cornea, giving the latter 

 its characteristic bright, reflecting appearance; no blood-vessels are visible through it here in 

 health. When the lids are shut, the space enclosed between their posterior surfaces and the 

 front of the eyeball is thus everywhere lined by conjunctiva, and is known as the con- 

 junctival sac. 



Not unfrequently the tendinous insertions of some or all of the recti muscles into the sclera 

 may be seen through the conjimctiva, each insertion appearing as a series of whitish parallel 

 lines running toward, but terminating about seven millimetres from, the corresponding corneal 

 border. 



The cornea appears as a transparent dome, having a curvature greater than that of the 

 sclera; the junction of the two unequally curved surfaces is marked by a shallow depression 

 running around the cornea, known as the scleral sulcus [sulcus sclera^]. In outline the cornea is 

 nearly circular, but its horizontal diameter is slightly greater than its vertical. Between it 

 and the iris a space exists, whose depth we (!an estimate roughly by looking at the eye from one 

 side; this space, or anterior chamber [camera oculi anterior] is occupied by a clear fluid, the 

 aqueous humour. Almost the whole anterior surface of the iris is visible, its extreme periphery 

 only being concealed by sclera. 



In coloin- th(! iris varies greatly in different individuals. Near its centre (really a little up 

 and in) a round hole exists in the iris, the black j>npil [pupilla], whose size varies considerably 

 in different eyes, and in the same eye according to temporary conditions, such as exposure to 

 light, etc. 



On the surface of the iris we see a number of ridges [plica) iridis] running more or less radially; 

 adjoining ones occasionally unite and interlace to some extent, so as to leave large depressed 

 meshes at intervals. These are the cri/pls of the iris. The radial ridges coming from the edge 

 of the pupil, and those coming from the more peripheral i)art of the iris, meet in a zigzag ele- 

 vated ridge concentric wifli tlie [)ui)il, called the corona rrulis, and by this ridge the iris is 

 roughly marked off info two uii('(|ual zones —an outer, the greater [annulus iridis major] and 

 an inner, the lesser [aiiriiiius iridis minor]. Tlic Ijorder n(>xt the i)upil [margo pupillaris] 

 is edged with small, roundish, bead-like prominences of a dark brown colour, separated 

 from one another by depressions, so that it presents a finely notched contour. Not infrequently, 

 in a light-colour(!(l iris, we may see the sphincter muscle through the anterior layers, in tlie form 

 of a ring about one millimetre in breadth around the pupil. The annulus iridis major may be 

 described as cons'sting of three parts: — (a) A comparatively smooth zone next the zigzag 

 ridge; (l>) a middle area, showing concentric but incoin|)letely circular furrows; (r) a small per- 

 ipheral darker part, presenting a sieve-like ajipearance. On the floor of the large depressed 



