ORGANS OF SENSE. 163 



each edge called the lachrymal papilla^ the point of which exhibits a minute 

 but distinct foramen, the punctum lachrymale^ which is the commencement 

 of a small canal, the lachrymal duct, by which the tears are conveyed into 

 the lachrymal sac and thence into the nose. From between the puncta the 

 palpebral fissure leads inwards and enlarges into a small triangular space, 

 the lacus lachrymalis, in the centre of which there is a red papilla, the 

 lachrymal caruncle, between the upper and lower lachrymal duct. The 

 upper eyelid is much deeper and more movable than the lower. The free 

 or ciliary margins of the palpebrae are thick, firm, and abrupt, cut o-flf hori- 

 zontally so as to meet closely by flat surfaces. 



The palpebnB are composed of skin, areolar tissue, an orbicular muscle, 

 cartilage or fibro-cartilage, with connecting fibrous membrane, glands, and 

 mucous membrane. The upper lid also has a special levator muscle, and 

 the free border of each is fringed with rows of hairs or cilia. 



The skin is continuous with that of the forehead and cheek, and is of 

 exceedingly delicate texture. The semilunar concentric folds, seen when 

 the eye is open, are effaced when the lids are closed. The areolar tissue is 

 very fine and loose, perfectly free from adipose structure. 



The cilia or eyelashes are stiff and strong, and have curved hairs placed 

 in three or four rows in the cutaneous edge of the free margin ; all are 

 curved or bent, the superior upwards, the inferior downwards, so that when 

 the lids are closed the convexities only touch, without the hairs becoming 

 entangled with each other. The cilia defend the eye from the admission of 

 particles of dust, minute insects, &c., and can also shade it in too strong a 

 light. Along the posterior or ocular edge of each free border is a row of 

 minute foramina, the openings of the ducts of the Meibomian glands. 



The orbicular or sphincter muscle is the next tissue in the palpebrae; it 

 also extends over the circumference of the orbit, superiorly into the super- ■ 

 cilia, and inferiorly into the cheeks. All the fibres are attached into the 

 internal tendon, the tendo oculi or palpebrarum ; this is inserted into the 

 nasal process of the maxillary bone, thence it is directed outwards in front 

 of the lachrymal sac, above its centre, and bifurcates. Each band incloses 

 a lachrymal duct, and is inserted into the inner extremity of each tarsal 

 cartilage beneath the punctum. This muscle closes the lids as a sphincter, 

 by depressing the upper one considerably and raising the lower one very 

 slightly, at the same time drawing it horizontally inwards. It supports the 

 globe of the eye, directs the lachrymal secretion into the puncta, cleans and 

 polishes the surface of the cornea ; it also serves to protect the eye from too 

 much light. 



The palpebral or tarsal cartilages are thin elastic plates covered by the 

 orbicularis muscle on the exterior surface. The ciliary margin in each is 

 abrupt and thickened, causing the firmness of the edges of the eyehds. The 

 orbital edge of each is thin, and attached to the base of the orbit by a fibrous 

 expansion termed the broad ligaments of the tarsus. The object of these 

 cartilages is to impart strength, pliancy, and elasticity to the palpebrse. 



The Meibomian glands or follicles appear on the eversion of the palpebrge, 

 as bony, pale yellow parallel ducts, leading from the free margin of each 



869 



