416 SPECIAL PHYSIOLOGY. 



brce. Each eyelid consists of a thin semilunar plate of soft fibro-car- 

 tilage, the tarsal cartilage, which gives it form and support; outside 

 this cartilage, is a thin, delicate, and very loose skin, destitute of fat, 

 and a few pale striated muscular fibres, belonging to the so-called 

 orbicularis muscle, together with some non-striated muscular fibres, 

 which are more numerous in the lower lid (H. Miiller); on its inner 

 surface, it is lined with mucous membrane. The cartilage of the upper 

 lid is larger and thicker than that of the lower lid, which forms merely 

 a narrow plate. The cartilages are connected at their orbital or at- 

 tached margins, with the periosteum of the circumference of the orbit, 

 by broad membranes called the fibrous membranes of the lids. At the 

 outer angles, these membranes are strengthened, and tie the outer 

 ends of the cartilages to the bone ; the inner ends of the cartilages are 

 connected with a short, strong, horizontal tendon, called the tendon of 

 the eyelids, or tendo oculi, which extends from the tips of the carti- 

 lages, to the inner wall of the orbit. The cartilages are kept in con- 

 tact with the eyeball, in all its various movements, by means of a small 

 muscle, named the tensor tarsi, placed behind the tendon of the eyelids. 



The levator palpebrce muscle, above mentioned, arises from the bot- 

 tom of the orbit, and passes forwards above the eyeball, to be inserted 

 into the posterior edge and surface of the upper tarsal cartilage ; it 

 pulls back this lid, and so uncovers the front of the eyeball. The 

 lower lid has no depressor muscle to lower it, but descends a little by 

 its own elasticity. According to Wagner, the unstriped muscular 

 fibres of the lids, also co-operate in opening the eyelids, being governed, 

 as shown by experiment, by the sympathetic nerve. The ordinary 

 closure of the eyelids is accomplished by the action of the part of the 

 orbicularis muscle which lies upon the eyelids ; their more forcible 

 closure, by the part of the same muscle which surrounds the orbit. 

 The levator palpebrae muscle is supplied by the third cranial nerve, 

 and the orbicular muscle by the seventh or facial nerve. 



The mucous membrane lining the inner surface of the eyelids, is 

 continuous with the skin at the free margins of the lids ; it is reflected 

 from the lids, over the fore-part of the eyeball, so as to connect these 

 two parts, whence it is called the conjunctiva; it is also prolonged into 

 various ducts and canals. Where it covers the anterior transparent 

 part of the eye, named the cornea, the conjunctiva is very thin, color- 

 less, and but slightly endowed with sensibility ; the part covering the 

 white portion of the eyeball, called the sclerotic coat, is somewhat 

 thicker. On the inner surface of the eyelids, it is much thicker, 

 highly vascular, very sensitive, provided with closely-set papilla, and 

 firmly adherent to the cartilages. It is covered by a many-layered 

 squamous epithelium. 



On the ocular surface of the tarsal cartilages, between them and 

 the conjunctiva, are situated the Meibomian glands. These are mod- 

 ified, and complex, sebaceous glands (Fig. 78), consisting of a series 

 of ducts, placed side by side, and perpendicularly to the margins of 

 the lids, each communicating with numerous lateral follicles or 

 crypts, b. They occupy little grooves on the inner surfaces of the 

 cartilages, and their ducts open, by minute orifices, a, on the free 



