THE ACCESSORY OBGANS OF VISION. 941! 



is succeeded by a fusiform fleshy mass, about three lines in diameter and an inch 

 long, embedded in adipose tissue ; it passes obliquely upwards and outwards on 

 the external face of the rectus muscle, and terminates in a thin flat tendon which 

 accompanies the upper belly of the superior oblique for a short distance, and 

 becomes confounded with the tendon of that muscle as it runs beneath the 

 superior rectus. It is supposed to be an accessory of the superior oblique, and 

 to regulate and facilitate the gliding of that muscle through the acute angle 

 formed by its pulley.) 



Protective Organs of the Eye. 

 1. The Eyelids (Figs. 502, 510). 



Preparation. — There is no difiBculty in studying the eyelids. Removing the skin carefully 

 allows the orbicularis to be seen ; in turning this up, the fibrous layer is found ; and if the 

 orbital process be removed by means of the saw, the levator palpebrse superioris is discovered 

 in cutting away the upper part of the ocular sheath. Lastly, on an eye extracted along with 

 the eyelids, the mode of union of these with the globe will be readily demonstrated. 



The surface of the eye is covered and protected in front by two movable 

 membranous curtains — the eyelids {palpebral) — one superior, the other inferior. 



Attached to the circumference of the orbit by their external border, the eye- 

 lids have a convex external face formed by the skin, and a concave internal face, 

 moulded on the anterior surface of the eye, and lined by the conjunctiva, which 

 is reflected above and beiow on the eyeball — the duplicatures constituting the- 

 superior and the inferior conjunctival {ox palpebral) sinuses. 



Each lid has also a free border opposed to that of its fellow, with which it- 

 unites at an angle by its extremities, so as to form two commissures (or canthi). 

 This border is shghtly bevelled on the inner side, and shows a series of small 

 openings — the excretory orifices of the Meibomian glands ; as well as a row of 

 erect hairs^ — the eyelashes. These wiU be described presently. 



When the two lids are closed by the approximation of their free borders, they 

 completely cover the eye, and form a narrow fissure comparable to a closed 

 button-hole. When they are separated, they circumscribe an oval space {fissura 

 palpebrarum), the greater axis of which is directed obliquely downwards, forwards^ 

 and inwards. The upper contour of this space — formed by the free margin of 

 the superior eyelid — is always more curved than the lower. The superior com- 

 missure (or canthus) has also been named the temporal angle of the eye. The 

 nasal angle, constituted by the inferior commissure, is always rounder than the 

 other ; it lodges the caruncula lacrymalis (in the lachus lachrymalis). 



Structure of the Eyelids. — A fibrous plate, terminated towards the free 

 border of the lid by a small tendinous arch named the tarsus ,• a sphincter muscle — 

 the orbicularis palpebroe — in contact with the fibrous membrane ; the levator pal- 

 pebrcB — a muscle partly lodged in the ocular sheath, and terminated anteriorly by 

 a very thin and wide expansion placed beneath the superior fibrous plate ; a 

 cutaneous envelope in two layers — an external, the skin, and an internal of 

 mucous membrane, the conjuncMva, joining at the free border of the lid ; — these 

 are the elements which enter into the composition of the protective coverings of 

 the eye. 



1. Fibrous Membrane. — Usually thicker in the lower than the upper lid, 

 this membrane is attached, by its adherent border, to the rim of the orbit, where 



