THE PARTS OF THE EYEBALL. 419 



rapid shutting and somewhat slower reopening of the lids, foreign 

 bodies are carried inwards, by a kind of sweeping movement, to the 

 lachrymal lake. The secretions of the conjunctiva and glandular 

 appendages of the eye, when flowing in moderate quantity, partly 

 evaporate, but are chiefly conveyed towards the lachrymal puncta, 

 whence the residuary fluid portion passes, partly by capillary attrac- 

 tion, and partly by the action of the orbicularis muscle, and of the 

 tensor tarsi muscle, hence called the muscle of the lachrymal sac, into 

 the lachrymal canals and sac, and thence, through the nasal duct, into 

 the nose. Deep and quick inspirations may likewise aid the descent 

 of the fluid, by an exhausting or sucking action. When the secretion 

 of the lachrymal glands is greater in quantity than can be carried 

 away by the lachrymal ducts, the overflow constitutes the tears. 



The secretions of the lachrymal gland and conjunctiva moisten the 

 surface of the eye, facilitate the movements of the eyeball, and, pre- 

 venting loss by evaporation, preserve the transparency of the so-called 

 cornea. The tears are the most abundant of these secretions ; but 

 after the loss of the lachrymal gland, the eye still remains moist. The 

 sebaceous secretion of the Meibomian follicles lubricates the margins 

 of the eyelids, prevents their adhesion, and protects them from the ac- 

 tion of the tears. An increased flow of tears is excited by the action 

 of strong light, by irritants operating on the conjunctival, nasal, and 

 lingual branches of the fifth cranial nerve, by vomiting, violent cough- 

 ing, and by mental emotions causing laughing or crying. 



The Eyeball 



The eyeball or globe of the eye, Fig. 80, is a strong closed membran- 

 ous sac, rudely compared to a globe, but in reality composed of a large 

 segment of one sphere, having a small segment of a lesser sphere af- 

 fixed to it prominently in front, c. The diameter of these two spheres 

 is about as 11 to 7. The eyeball is furnished with a number of small 

 muscles, which closely surround it, and is abundantly supplied with 

 vessels, lymphatics, and nerves. It is attached behind, to the optic 

 nerve, n, and is also maintained in position by its muscles, which pass 

 to it from the orbital walls. In front, the eyeball is free. It measures 

 about one inch in its antero-posterior diameter, and about one line 

 more, from side to side. The coats of the eyeball are partly transpa- 

 rent, partly opaque, the former occupying a portion of the front of the 

 eyeball, the latter the remainder of the globe. The former constitutes 

 the cornea ; the latter, of which only a part is visible, is the white coat 

 named the sclerotic. Within this coat, is spread out a black pigmentary 

 layer, named the choroid, and within this, the retina, the delicate ner- 

 vous expansion of the optic nerve. The interior of the globe is par- 

 tially divided into two parts by a perforated septum, named the iris, 

 and is occupied by certain transparent media, called humors. The 

 rays of light penetrate the transparent coat and media, to reach the 

 back part and sides of the interior of the eyeball ; passing through the 

 opening in the iris, and impinging on the retina, they form upon it 

 definite images of external objects. The effects of such impressions 



