1052 



SPECIAL SENSE ORGANS 



which processes pass to the brain in the optic nerve. The eyeball is a hollow spher- 

 ical structure, whose wall is formed externally by a fibrous tunic including the 

 sclera (the white of the eye) , and the cornea (the transparent area in the anterior 

 aspect of the eyeball). Internal to the tunic formed by these membranes is a 

 pigmented vascular membrane, the chorioidal membrane, of which the anterior 

 part forms the iris, or the coloured part of the eye. 



Within these tunics is formed a cavity, in which lies the crystalline lens of the 

 eye. In front and behind the lens are two chambers; that in front of the lens 

 contains the aqueous humour and that behind it the vitreous. 



The study of the eye is best undertaken by examining the eye in the living, and 

 subsequently by the dissection of specimens, and that order is followed in this 

 account. 



General Surface View 



The two eyes are situated nearly in the line where the upper and middle thirds of the face 

 meet ; thej' lie right and left of the root of the nose, the most prominent part of the front of each 

 globe being about 3 cm. (H in.) from the mid-line of the face. Each eye is overshadowed by 

 the corresponding eyebrow, and is capable of being concealed by its eyeUds, upper and lower. 



The orbital margin may be traced all round with the finger. At the junction of the medial 

 and intermediate thirds of the upper margin the supraorbital notch (incisura supraorbitaJis) 

 can usually be felt, and the supraorbital nerve passing through it can sometimes be made to 

 roll from side to side under the finger. The medial margin is the most difficult to trace in this 

 way, partly because it is more rounded oflf than the others, partly because it is bridged over by 

 a firm band (medial palpebral ligament), passing medially from the medial angle of the eyelids; 

 below this band, however, a sharp bony crest is felt, which lies anterior to the lacrimal sac. 

 Note how the eye is protected by the rim of the orbit, above and below; if we lay a hard flat 



Fig. 796.^ — View of the Eye wnn Eyelids Open. 

 Palpebra superior (pars tarsalis) 

 Cilia! Sulcus orbitopalpebralis superior 

 Sclera l ' ' Angulus oculi medialis 



' ' I Medial palpebral commissure 



1 



\ 



Iris 



^Pupil Caruncula lacrimalis 



Palpebra inferior 



body over the orbital opening, it will rest ui)on the upper and lower bony j)rominences, and will 

 not touch the surface of the globe. Medially, the eye is i)rotected from injury mainly by the 

 bridge of the nose; laterally it is most readily vulnerable, as here the orbital rim is comparatively 

 low. With one finger placed over the closed upper lid, press the eyeball gently backward into 

 the orbit, and observe the elastic resistance met with, due to the fact that the globe rests pos- 

 teriorly on a pad of fat. 



The si)ace between the free edges of the upper and lower lids is known as the palpebral 

 aperture [rima pal[)ebrarum]: it is a mere slit when the lids arc closed; but when they are open 

 its shape is, roughly, that of an almond lying with its long axis horizontal, and about thirty 

 milliriietrcs in Iciigtli. 



When the eyes are directed to an object stniight in front of them, this aperture is about 

 twelve niiliimetres wide, Ijut its width varies with ujjward and downward movements of the 

 eyeball, being greatest on looking strongly upward, diminishing gradually as the eye looks 

 progre.ssively lower. The angles formed by the meet ing of the lids at each end of the palpebral 

 aperture are named respectivc^ly the lateral and medial angles (or cantlii) [angulus oculi later- 

 alis, medialis], of which the lateral is shar[), while the medial is rounded otT. On a closer in- 

 spection, it will be found that, for the last five millimetres or so before reaching the medial angle 

 the edges of the lids run an almost parallel course, and are here devoid of lashes. Through the 

 o[)en palpebr.'il aperture the front of the eyeball comes into view, extending ciu'te to the lateral, 

 but not reaching as far as the medial, angle; just within the latter we find a small reddish promi- 

 nence, the lacrimal caruncle [caruncula lacrimalis); and between this and the eyeball a fold of 



