CHAP. XVII.J THE EYE. 15 



the force of blows directed towards that region. It is furnished 

 with muscles capable of moving it towards any side, and of pro- 

 truding or sinking it. It is likewise provided with moveable lids to 

 guard its exposed surface from mechanical injury, and its nerve 

 from the effects of excessive light; and with, a lachrymal apparatus, 

 by which the front of it is continually irrigated with a bland fluid. 

 In the globe of the eye itself we recognize, as the most essential 

 constituents, the expansion of the optic nerve, called the retina; and, 

 in front of this, the transparent refracting media which, as a whole, 

 transmit the light so as to bring its rays to a focus upon the nervous 

 sheet. The curved form of the retina, and the rounded figure of 

 the eye thence derived, are perfectly adapted to the curvatures of 

 the refracting media : so that, if the nervous lamina had assumed any 

 other shape, it would have been more or less out of focus, and vision 

 consequently have been indistinct. 



To maintain the figure of the retina, and to protect a part of 

 so much delicacy, in which the slightest change of form would be 

 attended with injury to the function, the whole is encased in a 

 dense tunic of great strength, termed the sclerotica (aic^pos, durus), 

 which is opaque, except in front, where it is modified in structure, 

 becomes perfectly transparent to allow the light to enter, and is 

 known as the cornea. Between the sclerotica and the retina is 

 interposed a layer of dark pigment, contained in a delicate mem- 

 brane termed the choroid. In front of the retina are the transparent 

 media. One of these (the vitreous body or humor) is contained 

 immediately within the cup which the retina forms, and appears 

 specially constructed to give it that necessary support inside, which 

 the sclerotica furnishes on the outside. The vitreous body occupies 

 four-fifths of the whole globe. Imbedded in its anterior part is a 

 double convex lens (the crystalline lens or body), which comes nearly 

 up to the cornea; leaving, however, a small cavity containing a 

 watery fluid, the aqueous humor, between itself and that transparent 

 part of the external case. Across this cavity, and dividing it into 

 an anterior and posterior chamber, hangs a vertical curtain-like 

 process of the choroid, called the iris, perforated in the centre by 

 an aperture, the pupil, for the admission of light to the interior, and 

 contractile under the influence of light on the retina, in order that it 

 may regulate the amount of light entering the organ. The perfect 

 fluidity of the aqueous humor is a provision to allow of the ex- 

 pansion and contraction of the pupil, and of the movements of the 

 lens itself towards 'or from the cornea. 



The human eye would be nearly globular were it not that the 



