ORGANS OF THE SENSES. THE EYE. 705 



IY. ORGANS OF THE SENSES. 



In this place it is intended to describe the organs of sight, hearing and 

 smelling, which, considered with reference to their anatomy and develop- 

 ment, are regarded as the higher organs of special sense. The description 

 of the organ of touch is given along with the skin in the histological part 

 of the work, and that of the organ of taste along with the descriptive 

 anatomy of the digestive system. 



THE EYE. 



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

 of the eye, a spheroidal structure enclosed by strong membranous coverings, 

 receiving the optic nerve posteriorly, and containing the sensitive termina- 

 tions of that nerve, together with a series of transparent media, which con- 

 stitute an optical instrument of variable focus, through which the rays of light 

 are transmitted to the sensitive part, and so brought into focus as to form 

 upon it a distinct inverted image of the objects from which they proceed. 

 Eut there are likewise various structures external to the eyeball which con- 

 tribute to the production of perfect vision, such as the straight and oblique 

 muscles by which the eyeball is moved in different directions, and the various 

 supporting and protective structures known as appendages of the eye (tuta- 

 mina oculi), including the eyebrows, eyelids, and conjunctiva, and the lachry- 

 mal apparatus. 



APPENDAGES OP THE EYE. 



THE EYELII>S AND CONJUNCTIVA. 



The eyelids (palpebrse) are moveable folds of integument, strengthened 

 toward their margins by a thin lamina of cartilage. The mucous membrane, 

 which lines their inner surface, and which is reflected thence in the form of 

 a pellucid covering on the surface of the eyeball, is named membrana con- 

 junctiva. 



The upper lid is larger and more moveable than the lower : the trans- 

 parent part of the globe is covered by it when the eye is closed ; and the eye 

 is opened chiefly by the elevation of this lid by a muscle (levator palpebrse) 

 devoted exclusively to this purpose. The eyelids are joined at the outer and 

 inner angles (canthi) of the eye. The interval between the angles, fissura 

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

 extent, the size of the globe being nearly the same, gives the appearance of 

 a larger or a smaller eye. The greater part of the edge of each eyelid is flat- 

 tened, but towards the inner angle it is rounded off for a short space, at the 

 same time that it somewhat changes its direction ; and, where the two differ- 

 ently formed parts join, there exists on each lid a slight conical elevation 

 papitta lachrymalis, the apex of which is pierced by the aperture or punctum 

 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 lachrymalia, there 

 intervenes a vertical fold of conjunctiva, the plica semilunaris, resting on 

 the eyeball ; while, occupying the recess of the angle internal to the border 

 of this fold, is a spongy-looking reddish elevation, formed by a group of 



3 A 



