ANIMAL MECHANISM. 107 



body. This form gives them the faculty of surveying the 

 expanse of a field, at once. Farmers are familiar with 

 the circumstance that the ox, without being obliged to 

 undergo the fatigue of circuitous inarches, walks directly 

 to the best feed in the whole lot, provided the enclosure 

 be a plain. See fig. 3, letters ee, and k. Fig. 4, let- 

 ters c c. 



CILIARY PROCESSES. 



Ciliary Processes. Directly behind the iris, is a second 

 curtain, having a central hole through it, corresponding 

 with that through the first curtain, but nearly as large as 

 the whole diameter of the lens. All the luminous rays 

 which are converged by the convexity of the cornea, which 

 is, in effect, a plane convex lense, cannot enter through 

 the pupil ; many of them strike the plane of the iris, and 

 are reflected back, as on a looking-glass, without pene- 

 trating its substance. If an) rays were to get through, 

 by such an irregular process, it would produce great con- 

 fusion, by destroying the outline and vividness of the 

 image previously made on the visual nerve, through the 

 natural opening. To prevent such mishaps, the paint on 

 the back of the iris is to absorb such rays as are not 

 reflected, and have a tendency therefore to pass onward. 

 Nature, as though fearful that circumstances might so 

 alter the condition of the pigment,* as that some light, 

 notwithstanding this precaution, might penetrate, has 

 interposed this second veil, solely it is supposed to stop 

 all wandering rays. This ciliary curtain presents three 

 thicknesses, and lastly has a thick coat of black paint on 

 its back. In order to give it treble security, as it regards 

 thickness, it is plaited like the folds of a ruffle. There 

 are seventy folds in the human eye, of equal width, nicely 

 laid, one over the other. A part so highly important, 

 cannot be overlooked in studying the philosophy of vision. 



* Pigment paint. 



