12 A TYPICAL VERTEBRATE EYE: THE HUMAN 



It needs to be made clear at this point that the adjective 'elastic', 

 wherever it is appUed to an ocular structure, means 'springy' rather than 

 'easily stretched' — thus Descemet's membrane, the lens capsule, the 

 zonule fibers and so forth are elastic in the sense of a celluloid strip, 

 not of a rubber cord. 



The Intra-Ocular Fluids — The fibrous tunic is normally kept dis- 

 tended to the point of rigidity by the pressure of fluid secreted within 

 the eye. This fluid, the aqueous humor, is continuously produced at a 

 slow rate and drained out of the eyeball into the blood stream by a 

 complex arrangement which is so regulated that the intra-ocular pressure 

 remains roughly constant at about 25 millimeters of mercury. Half of 

 this internal pressure is created by the external pressure of the extra- 

 ocular muscles and if both these and the blood-vessels leading to the eye 

 are severed, the mtra-ocular pressure falls to about 10 millimeters of 

 mercury. Overproduction of aqueous humor or any chemical, mechan- 

 ical, or pathological upset in the drainage system will lead to a painful 

 rise in pressure, the condition being known as glaucoma. If the pressure 

 is unrelieved, it clouds the cornea and injures the retina, and the end 

 result is blindness. 



The greater portion of the intra-ocular fluid, occupying the large 

 chamber in the back of the eye, is rendered gelatinous by the addition 

 to it of proteins secreted during development by the retina. This mass 

 of gelated aqueous is called the vitreous (= glassy) body, or vitreous 

 humor (Fig, 3; Fig. 5, r). It is relatively permanent and in the fully 

 grown eye it is fixed in amount, so that any portion of it which is lost 

 through a wound is replaced only by watery aqueous humor. It is mostly 

 to the unmodified aqueous, in the front of the eye, that fresh fluid is 

 constantly added; and it is with the liquid aqueous that the pressure- 

 regulatory drainage mechanism — the canal of Schlemm (Fig. 3; Fig 5, 

 sc) communicates in an indirect way. 



So far as the human eye itself is concerned, there is no powerful 

 reason why the material which fills the chambers of the eye should be 

 of two kinds — liquid anteriorly and semi-solid posteriorly. But in the 

 forebears of the fishes, which invented the vertebrate eye, the material 

 near the cornea had to be kept fluid so that the lens could be readily 

 moved in accommodating the focus of the eye to different distances, 

 and the lens would have dropped back into the globe if there were 

 only liquid behind it. In the higher vertebrates, the lens is not changed 



