942 THE SENSORY APPARATUSES. 



it is continuous with the periosteum and the fibrous wall of the ocular sheath. 

 Its free border is margined by the tarsus. 



2. Tarsus. — This is a fibrous lamella that forms a solid frame for the free 

 border of the lid. It is elongated, narrow at its extremities, thin at its fixed 

 border — where it is confounded with the fibrous membrane — and channeled on its 

 inner face by several transverse parallel grooves which lodge the Meibomian 

 glands. This small fibrous arc regulates the contraction of the orbicularis 

 muscle, and prevents the lid being drawn into wrinkles ; by the rigidity it bestows 

 on the eyelids, it allows these to meet — border to border — without puckering, 

 when the muscle is in action. 



3. Orbicular Muscle of the Eyelids (orbicularis palpebrarum, musculm 

 ciliaris Riolani). — For a description of this muscle, see Myology, p. 279. 



4. Elevator Muscle of the Upper Eyelid, or Orbito-palpebralis 

 {Levator palpebrce superioris). — When the ocular sphincter ceases to contract, the 

 lower eyelid droops from its own weight ; the upper lid, however, requires some 

 special muscular agency to raise it, and this it finds in the levator palpebrae. 

 This is a very thin, narrow, fleshy band, lodged in the ocular sheath with the 

 other muscles of the eyeball, and is related to the superior rectus, the com-se of 

 which it follows. On reaching the lachrymal gland, it expands into a wide 

 aponeurotic membrane that passes between the conjunctiva and the fibrous plate 

 of the eyelid, and terminates on the tarsus. (Besides ending in the fascia jjalpe- 

 bralis, some of the fibres — enclosing smooth muscular fibres — pass on to the upper 

 margin of the tarsus to constitute Miiller's muscle — or palpebralis superior. 

 Similar smooth fibres in the lower lid, form a palpebralis inferior.) 



It will be seen that this muscle is inflected on the eyeball in a pulley-like 

 manner, and it is owing to this disposition that it has the power of raising the 

 lid. If the eyeball were not present, the muscle would draw the free margin of 

 the hd towards the back of the orbit, instead of elevating it. 



5. Integuments of the Eyelids. — The different layers enumerated are 

 comprised between two tegumentary folds— the skin and conjunctiva — which are 

 continuous at the border of the eyelids. We will examine these, with their 

 appendages — the eyelashes and Meibomian glands. 



a. Skin. — Intimately adhering, by its inner face, to the orbicularis muscle, 

 this membrane is thin (smooth), and covered with numerous fine short hairs. In 

 the foetus, it shows at the orbital arch — when the skin everywhere else is nude — 

 a well-marked semicircle of hairs — ^the eyebrow. Fat is never found beneath it. 



b. Conjunctiva. — The conjunctiva, as its name indicates, joins the eyelids to 

 the eyeball. Very fine and highly vascular, this mucous membrane is a continua- 

 tion of the skin at the border of the lids, lines the inner face of each of them, 

 envelopes the anterior portion of the membrana nictitans in a particular fold, 

 covers the caruncula lachrymalis, and enters the puncta ; it is then reflected, at 

 the adherent border of the eyelids, on to the eyeball, extending over the sclerotic 

 and terminal aponem-otic expansion of the recti muscles. On arriving at the 

 margin of the cornea, it is impossible to trace it further ; though it is represented 

 by the thin layer of pavement epithelium already described. At the surface of 

 the lachrymal caruncle, it shows some very fine hair-bulbs. It possesses some 

 papillae (on the palpebral portion only, the ocular reflection being thinner, and 

 having none of these nervous processes), and tubular and aggregate glands, as 

 well as closed follicles. We have found large numbers of the latter, the volume 

 of which was considerable ; they form a corona around the cornea. 



