66 EVOLUTION OF THE HUMAN EYE 



morphologically the same as the endothelial cells found in 

 the spaces of the lamina suprachoroidea and in the pectinate 

 ligament, and that the development of the ciliary muscle 

 is due to the proliferation and lengthening out of such 

 endothelial cells. 



Anatomists have frequently described Descemet's mem- 

 brane as the uveal layer of the cornea, and I think such 

 a description is both accurate and useful. Descemet's 

 membrane is composed of the same elastic tissue as that 

 of the pectinate ligament, both staining in a similar way 

 with acid orcein. Elastic fibers are also found in the lamina 

 suprachoroidea. The elastic tissue of Descemet's mem- 

 brane has been proved to be produced as a kind of secretion 

 of the endothelial cells lining it. In the human fetus, before 

 the iris is formed, and before there is any anterior chamber, 

 two parallel rows of cells are seen lying interposed between 

 the cornea and the anterior capsule of the lens. The anterior 

 chamber is developed as a space between these two rows of 

 cells, the anterior of which becomes the lining endothelium 

 of Descemet's membrane, and the posterior the endothelium 

 on the surface of the iris and of the pupillary membrane. 

 It may therefore be claimed that not only does Descemet's 

 membrane represent the uveal layer in the cornea, but 

 that the anterior chamber is analogous to an opened-out 

 and enlarged potential space of the lamina suprachoroidea. 



These morphological analogies of the fibers of the ciliary 

 muscle and its surroundings are of considerable assistance 

 in understanding the distribution of its fibers in the human 

 eye. Having recognized that the fibers of which the muscle 

 is composed are developed from cells contained in spaces on 

 the outer surface of the ciliary body, it becomes evident 

 that the shape and direction of these muscle fibers will 

 depend on that of the spaces in which they are formed. 



