THE ACCESSORY ORGANS OF VISION. »43 



c. Eyelmhes. — These are two rows of hairs {cilia) implanted in the free 

 border of the lids, and destined to prevent the entrance of dust and small 

 particles of foreign matter into the eye. They are much longer, and more 

 abundant and stronger, in the upper than the lower lid, their presence there 

 being more necessary, as extraneous particles are most likely to enter the eye when 

 falling. But if the eyelashes of the lower lid are few and rudimentary, this is 

 compensated for by the presence on its surface of some long bristly haira, 

 scattered here and there, and exactly like the tentacula of the lips. 



Like all hairs, without exception, the eyelashes are flanked at their base by 

 two or three small sebaceous glands, the duct from which opens into their 

 follicle. 



d. Meibomian glands. — These are little acinous bodies, analogous to sebaceous 

 glands, wliich open alternately into a common, and very long excretory canal. 

 They are lodged in the transverse grooves observed on the inner face of the tarsal 

 ligaments. The unctuous matter they secrete is thrown out on the free border 

 of the lids, and enables these to retain the tears more easily within the ocular 

 cavity. In sick animals, this secretion accumulates at the canthi and base of the 

 lids. (Each gland consists of a central tube, with a number of openings round 

 its sides leading to short caecal dilatations. The secretion also facilitates the 

 movements of the lids.) 



G. Vessels and Nerves of the Eyelids. — These membranous curtains 

 receive their blood, for the most part, by the supra-orbital and lachrymal 

 arteries, and the orbital branch of the superior dental artery. The terminal 

 extremities of the three sensitive nerves of the eye, formed by the ophthalmic 

 branch of the fifth pair and the orbital filaments of the superior maxillary 

 branch, ramify in them. The anterior auricular nerve causas the orbicularis 

 muscle to contract. The motor filaments of the levator palpebrae are derived 

 from the third pair. 



(The blood-vessels of the eyelids proceed from those which pass from the 

 outer and inner angle of the eye ; they form an arch on the inner margin of the 

 eyelid — the arcus tar sens externus. After supplying the tissues, some of these 

 vessels anastomose with the arteria ciliaris antira. The ocular conjunctiva has 

 generally few blood-vessels visible in health ; when inflamed, however, it becomes 

 intensely red and vascular. 



The lymphatics form a dense network in the tarsal conjunctiva ; those in 

 this membrane in front of the sclerotic and around the margin of the cornea, 

 probably join the small canals of the latter. The tunica piopria of the eyelid 

 has many lymph-cells, and in some animals — as Ruminants — they form small 

 lymphoid glands. 



The nerves of the conjunctiva form a rich plexus on the margin of the eyelid, 

 and terminate in small oval enlargements or end knobs — the corpuscles of Krause 

 —which are more particularly observed beneath the corneal epithelium.) 



2. Membrana Nictitans. 



This organ, which is also named the third eyelid, ivinlciny eyelid, etc., is placed 

 at the greater (inner) angle of the eye, whence it extends over the eyeball to 

 relieve it from foreign bodies which may fall upon it. 



It has for its framework a fibro-cartilage — reticulated or elastic — irregular 

 in shape, thick and nearly prismatic at its base, and thin anteriorly, where it is 

 covered by the conjunctiva ; it is continued, behind, by a strong adipose cushion, 



