THE ACCESSORY ORGANS OF VISION. 833 



glands, which open alternately into a common, 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 csecal dilatations. The 

 secretion also facilitates the movements of the lids.) 



6. 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 endows the orbicularis 

 muscle with contractility. The motor filaments of the levator palpebrse are 

 derived from the third pair. 



2. Membrana Nictitans. 



" This organ, which is also named the third eyelid, winking eyelid, etc., is 

 placed at the great (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 frame work a fibro-cartilage elastic irregular in shape, thick and nearly 

 prismatic at its base, and thin anteriorly where it is covered by the conjunc- 

 tiva ; it is continued, behind, by a strong adipose cushion, which is insinuated 

 between all the muscles of the eye, and to which it is loosely attached. No 

 muscle directly concurs in the movements of this body : they are entirely 

 mechanical. When the eye is in its usual position, there is only per- 

 ceived the fold of conjunctiva that terminates it in front ; the remainder is 

 concealed in the fibrous case of the eye. When, however, the latter is 

 withdrawn into the orbit by the contraction of its recti muscles, the globe 

 compresses the fatty cushion belonging to the cartilage; this cushion, 

 pressing outwards, pushes the membrana before it, and the latter then entirely 

 conceals the whole front of the globe. This movement is instantaneous, 

 but it may be momentarily fixed by pressing gently on the eye when the 

 animal retracts it within the orbital cavity. 



" The use of the membrana is, as will be seen from the above, to main- 

 tain the healthy condition of the eye, by removing any matters that have 

 escaped the eyelids; and what clearly demonstrates this function, is the 

 inverse relation that always exists between the development of this body, 

 and the facility with which animals can rub their eyes with their anterior 

 limbs. So it is that, with the Horse and Ox, whose thoracic member cannot 

 be applied to this purpose, the membrana is very developed ; and in the Dog, 

 which may use its paw to some extent when it requires to brush its eye, it 

 is smaller ; in the Cat it is still less ; while in the Monkey and in Mankind, 

 whose hands are perfect, it is rudimentary. In tetanus, the membrana 

 nictitans often remains permanently over the eye, in consequence of the 

 continued contraction of the recti muscles." F. Lecoq. 



(Towards the middle of the outer face of the membrana is a small 

 yellowish-red gland, the gland of Harder, covered by a strong fibrous 

 membrane, and surrounded by adipose tissue ; it secretes a thick unctuous 

 matter, which escapes by two or three small apertures on the inner face of the 

 membrana.) 



