790 



CHAPTER XT. 

 DISSECTION OF THE EYE. 



Situation of 

 the eyeball ; 



surrounding 

 sheath. 



Parts in 

 front of it. 



The dissec- 

 tion to be 

 made on the 

 eye of the 

 ox. 



Detach the 

 muscles. 



Form of the 



ball. 



Position of 

 optic nerve. 



Diameter. 



Composi- 

 tion; 



number of 

 coats, 

 and central 

 parts. 



Dissection. 



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

 Supported 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 or capsule of Tenon) isolates the ball, so as to 

 allow free movement. 



Two lids protect the eye from external injury, and regulate the 

 amount 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 

 structure may be learnt on the eye of the pig or ox. 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, will 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. 288) consists of two parts, which differ in 

 appearance, viz., an opaque white posterior portion (sclerotic), form- 

 ing about five-sixths of the whole, and a smaller transparent piece 

 (cornea) in front ; these two parts are segments of different sized 

 spheres, the anterior 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 ciliary vessels and nerves enter. 



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

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



The organ of vision is composed of central transparent parts, 

 with enclosing membranes or coats. The coats, placed one within 

 another, are named sclerotic, choroid, and retina. The transparent 

 media in the interior are likewise three, viz., the lens, the aqueous 

 humour, 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 recognise the arrangement of 

 the parts mentioned above, with their strength and appearance, and 

 will be better prepared to follow the directions that are after- 

 wards given. 



