EYEBROWS. EYELIDS. 503 



except in the dimness of evening or at night. In the manufacture of 

 optical instruments care is taken to colour their interior black with the 

 same object, the absorption of scattered rays. 



The transparent lamellated cornea and the humours of the eye have 

 for their office the refraction of the rays in such proportion as to direct 

 the image in the most favourable manner upon the retina. Where 

 the refracting medium is too great, as in over convexity of the cornea 

 and lens, the image falls short of the retina (myopia, near-sightedness) ; 

 and where it is too little the image is thrown beyond the nervous 

 membrane (presbyopia, far-sightedness). These conditions are recti- 

 fied by the use of spectacles, which provide a differently refracting 

 medium externally to the eye, and thereby correct the transmission of 

 light. 



APPENDAGES OF THE EYE. 



The Appendages of the eye (tutamina oculi) are the eyebrows, eye 

 lids, eyelashes, conjunctiva, caruncula lachrymalis, and the lachryma 

 apparatus. 



The Eyebrmi's (supercilia) are two projecting arches of integument 

 covered with short thick hairs, which form the upper boundary of the 

 orbits. They are connected beneath with the orbiculares, occipito- 

 frontales, and corrugatores superciliorum muscles ; their use is to 

 shade the eyes from a too vivid light, or protect them from particles of 

 dust and moisture floating over the forehead. 



The Eyelids (palpebrce) are two valvular layers placed in front of 

 the eye, serving to defend it from injury by their closure. When 

 drawn open they leave between them an elliptical space, the angles of 

 which are called canthi. The outer canthus is formed by the meeting 

 of the two lids at an acute angle. The inner canthus is prolonged for 

 a short distance inwards towards the nose, and a triangular space is 

 left between the lids in this situation, which is called the locus lachry- 

 malis. At the commencement of the lacus lachrymalis upon each of 

 the two lids is a small angular projection, the lachrymal papilla or 

 tubercle; and at the apex of each papilla a small orifice (punctum 

 lachrymale), the commencement of the lachrymal canal. 



The eyelids have, entering into their structure, integument, orbicu- 

 laris muscle, tarsal cartilages, Meibomian glands, and conjunctiva. 



The tegumentary areolar tissue of the eyelids is remarkable for its 

 looseness and for the entire absence of adipose substance ; it is parti- 

 cularly liable to serous infiltration. The fibres of the orbicularis 

 muscle covering the eyelids, are extremely thin and pale. 



The Tarsal cartilages are two thin lamellae of fibro-cartilage about 

 an inch in length, which give form and support to the eyelids. The 

 superior is of a semilunar form, about one-third of an inch in breadth 

 at its middle, and tapering to each extremity. Its lower border is 

 broad and flat, its upper is thin, and gives attachment to the levator 

 palpebrae and to the fibrous membrane of the lids. 



