vi DIOPTRIC MECHANISM OF THE EYE 277 



Embryology and a recent Monograph by Cirincione). The vitreous 

 body, the uvea (choroid, ciliary zone, iris), and the outer coat 

 (sclerotic and cornea) are mesodermal in origin; the inner coat 

 alone is ectodermal in origin, and of the parts which constitute 

 it (retina, zonule, crystalline lens) only the inner layer of the 

 retina had in the adult the character of a sensorial tissue, 

 composed, as we shall see, of specific neural elements the outer 

 layer remains as a single stratum of pigmented epithelial cells, in 

 close functional relation with the nerve-cells of the retina. 



Giambattista della Porta (1589) first discovered the optical 

 instrument known as the camera obscura, which he compared to 

 the eye, noting the correspondence of its parts : the convergent lens 



Fio. 116. Chproid membrane and iris exposed by removal of the sclerotic and cornea. Twice the~ 

 natural size. (Zinn.) a. part of the sclerotic turned back ; b, ciliary muscle ; c, iris ; e, 

 one of the ciliary nerves ; /, one of the vasa vorticosa or choroidal veins. 



of the camera obscura corresponds to the crystalline lens of the 

 eye ; the diaphragm corresponds to the iris, while, the surface on 

 which the reduced and inverted image of external objects is formed 

 in the camera corresponds to the retinal surface of the eye. The 

 dioptrics of the eye were first worked out systematically by 

 Kepler (1602). But neither della Porta nor Kepler saw the 

 formation of images on the retina. This was first demonstrated 

 by Christoph Schemer, a Jesuit Father, who observed it on the 

 excised eye of freshly killed animals (1609), and also on the 

 human eye when the retina was exposed at the back of the globe 

 (1625). 



The simplest method of seeing the formation of images at the 

 back of the eye is that of Magendie (1836). He dissected the eye , 

 of an albino rabbit, and tnen, after removing all portions of the 



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