PARTS FOR THE PROTECTION OF THE EYEBALL 707 



of the orbit, passes forward above the eyeball and spreads into a thin 

 tendon, which is inserted into the anterior surface of the superior tarsal 

 cartilage. Its action is to raise the upper lid. This muscle and its 

 relations are shown in Fig. 179(9, IO > IO )> P a g e 699. 



In the act of opening the eyes the levator muscles alone are brought 

 into play. Closing the lids is accomplished by the orbicular muscles. 

 Both these sets of muscles act to a great extent without the interven- 

 tion of the will. The eyes are kept open almost involuntarily, except 

 in extreme fatigue ; although when the will ceases to act the lids are 

 closed. Nevertheless there is hardly a conscious effort usually in keep- 

 ing the eyes open, and an effort is required to close the eyes. During 

 sleep the eyes are closed and the globes are turned upward. The con- 

 tractions of the orbicular muscles which take place in winking usually 

 are involuntary. This act occurs at short intervals, and it is useful in 

 spreading the lachrymal secretion over. the exposed portions of the 

 globes. The action of both sets of muscles ordinarily is simultaneous, 

 although they may be educated so as to close one eye while the other is 

 open. The action of the orbicularis is so far removed from the control 

 of the will, that when the surface of the globe is touched or irritated or 

 when the impression of light produces intense pain, it is almost impos- 

 sible to keep the eye open. 



Conjimctival Mucous Membrane. The entire inner surface of the 

 upper and lower eyelids is lined with a mucous membrane which is re- 

 flected forward from the inner periphery of the lids over the eyeball. 

 The membrane lining the lids is called the palpebral conjunctiva, and 

 that covering the eyeball, the ocular conjunctiva. The latter presents a 

 sclerotic and a corneal portion. The conjunctiva presents a superior 

 and an inferior fold, where it is reflected over the globe. In the supe- 

 rior conjunctival fold, are glandular follicles, or accessory lachrymal 

 glands, which secrete a certain portion of the liquid that moistens the 

 surface of the eyeball. These usually are described as forming a part 

 of the lachrymal gland. At the inner canthus there is a vertical fold, 

 the plica semilunaris, with a reddish spongy elevation at its inner por- 

 tion, called the caruncula lacrymalis. The caruncula presents a collec- 

 tion of follicular glands, with a few delicate hairs on its surface. The 

 conjunctiva is continuous with the membrane of the lachrymal ducts, of 

 the puncta lacrymalia and of the Meibomian glands. Beneath the con- 

 junctiva, except in the corneal portion, is a loose connective tissue. 



The palpebral conjunctiva is reddish, thicker than the ocular portion, 

 furrowed, and presents small isolated papillae near the borders of the 

 lids, which increase in number and size toward the folds. This portion 

 of the membrane presents large capillary bloodvessels and lymphatics 



