446 APPENDAGES OP THE EYE. 



the nasal branch of the ophthalmic, and the ciliary nerves from the 

 ciliary ganglion. 



Observalions. The sclerotic is a tunic of protection, and the 

 cornea a medium for the transmission of light. The choroid sup- 

 ports the vessels destined for the nourishment of the eye, and by its 

 pigmentum nigrum absorbs all loose and scattered rays that might 

 confuse the image impressed upon the retina. The iris, by means 

 of its powers of expansion and contraction, regulates the quantity of 

 light' admitted through the pupil. If the iris be thin, and the rays of 

 light pass through its substance, they are immediately absorbed by 

 the uvea ; and if that layer be insufficient, they are taken up by the 

 black pigment of the ciliary processes. 



In Albinoes, where there is an absence of pigmentum nigrum, the 

 rays of light traverse the iris and even the sclerotic, and so over- 

 whelm the eye with light, that sight is destroyed, 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, i 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 arc rectified 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, 

 eyelids, eyelashes, conjunctiva, caruncula lachrymalis, and the 

 lachrymal apparatus. 



The Eyebrows (super cilia) are two projecting arches of integu- 

 ment 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 (jpalpebrce) 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 canthL 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 Jacus lachrymalis. At the commencement of the lacus 



