BALL OF THE EYE. 655 



CHAPTER X. 



DISSECTION OF THE EYE. 



THE eyeball is the organ of vision, and is lodged in the orbit. Sup- 

 ported in that hollow on a mass of fat, it is surrounded by muscles which 

 impart movement to it ; and a thin membrane (tunica vaginalis oculi) 

 isolates the ball, so as to allow free movement. 



Two lids protect the eye from external injury, and moderate the degree 

 of light admitted into the interior ; and the anterior or exposed surface is 

 covered by a mucous membrane (conjunctiva). 



Directions. In the absence of specimens of the human eye, the struc- 

 ture may be learnt on the eye of the ox or pig. Let the student procure 

 half a dozen eyes of the ox for the purpose of dissection. One or two 

 shallow basins will be needed ; and some wax or tallow in the bottom of 

 one, or in a deep plate, may be useful. 



Dissection. To see the general form of the ball of the eye, and the 

 outer surface of the external coat, the attachments of the different muscles 

 are to be taken away ; and the loose mucous membrane is to be removed 

 from the front. 



The ball of the eye (fig. 227) is roundish in form and consists of two 

 parts, which differ in appearance, viz., an opaque posterior portion, form- 

 ing five-sixths of the whole, and a smaller transparent piece (cornea) in 

 front ; these two parts are segments of different-sized spheres, the an- 

 terior belonging to the smaller sphere. To the back of the eye the optic 

 nerve is attached, rather to the inner side of the axis of the ball ; and 

 around it nutritive vessels and the nerves enter. 



The antero-posterior diameter of the ball amounts to nearly an inch 

 (y^ths), but the transverse measures an inch. 



The organ of vision is composed of central transparent parts, with in- 

 closing membranes or coats. The coats, posited one within another, are 

 named sclerotic, choroid, and retina. The transparent media in the in- 

 terior are likewise three, viz., the lens, the aqueous humor, and the vitreous 

 body. 



Dissection. To obtain a general idea of the structures to be dissected, 

 the student may destroy one eyeball by cutting through it circularly : he 

 will then be able to recognize the arrangement of the parts mentioned 

 above, with their strength and appearance : and will be better prepared 

 to follow the directions that are afterwards given. 



FIBROUS COAT OF THE EYEBALL. The outer casing of the eye consists 

 of an opaque hinder part called sclerotic, and of an anterior transparent 

 portion, the cornea. 



The SCLEROTIC (cornea opaca) is the firm, whitish, and opaque portion 

 of the external stratum of the eyeball, which supports the more delicate 

 structures within. 



Dissection. To examine the inner and outer surfaces of this layer it 

 will be necessary to cut circularly with a scissors through the cornea close 



