THE EYE. 



ORGANS OF THE SENSES. 



lu this place will be described the organs of sight, hearing, and 

 smell — the higher organs of special sense. The description of the 

 organ of touch is given with the skin, and that of the organ of taste 

 with the tongue. 



THE EYE. 



The organ of vision, strictly speaking, consists only of the ball or 

 globe of the eye ; but connected with the eyeball externally are muscles, 

 nerves, and blood-vessels, elsewhere described, as well as other parts 

 specially destined for its protection, and known as the appendages of 

 the eye (tutamina oculi), of which an account will first be given. 



THE EYELIDS ANT) CO^STJUj^TCTIVA. 



The eyelids (palpchrce) are moveable portions of integument, strength- 

 ened toward their margins by a thin lamina of dense fibrous tissue. A 

 mucous membrane lines their inner surface, and is reflected thence in 

 the form of a pellucid covering on the surface of the eyeball. This is 

 named the conjunctival membrane or conjunctiva. 



The upper lid is larger and more nioveable than the lower, all the 

 transparent part of the globe being covered by it when the eye is closed ; 

 it is chiefly by the elevation of this lid that the eye is opened, the 

 movement being effected by a muscle (levator palpebr?e) devoted exclu- 

 sively to this purpose. At the outer and inner angles {canthi) of the 

 eye the eyelids are united. The interval between the angles — fissura 

 palpebrarum — varies in length in different persons, and, according to its 

 extent, gives the appearance of a larger or a smaller eye, the size of the 

 globe being nearly the same. The greater part of the edge of each eyelid 

 is flattened, but towards the inner cauthus it is rounded off" for a short 

 space, at the same time that it somewhat changes its direction ; where 

 the two diffbrently formed parts join, there exists on each lid a slight 

 conical elevation — 2)a2)iUa lachrymaUs — the apex of which is pierced by 

 the aperture ov punciimi of the corresponding lachrymal canalicule. 



In the greater part of their extent the lids are applied to the surface 

 of the eyeball ; but at the inner canthus, opposite the puncta lachry- 

 malia, there intervenes a vertical fold of conjunctiva, the plica semilu- 

 naris, which rests on the eyeball ; whilst, occupying the recess of the 

 angle at the border of this fold, is a spongy-looking reddish elevation, 

 formed by a group of sebaceous glands which open into the follicles of 

 very fine hairs. It is named the caruncula lachrymaUs. The plica 

 semilunaris is the rudiment of the third eyelid (membrana nictitans) 

 found in many animals. It contains a small amount of nlain muscular 

 tissue (H. Miiller). 



Structure of the lids. — The skin covering the eyelids is thin and 

 delicate, and covered with excessively fine, downy hairs ; at the line of 

 the eyelashes it joins the conjunctival mucous membrane which lines 

 the inner surface of the lids. The cutis vera is remarkable for contain- 

 ing ramified pigment cells. Beneath the skin, and between it and 

 the conjunctiva, the following structures are successively met with, viz. : 



