64 EVOLUTION OF THE HUMAN EYE 



are called the pillars of the iris. These consist of prolon- 

 gations forward of the stroma of the iris to the back of the 

 cornea, to which they are adherent just where Descemet's 

 membrane terminates (Figs. 11 and 12). It is these bands 

 of iris tissue, which are not present in man's eye, that give 

 the toothed appearance to the ligament, and which led to 

 its being compared to a comb. It is the disappearance 

 of these pillars of the iris in man's eye which has allowed 

 the anterior part of the ciliary body to become displaced 

 backward. As the angle of the anterior chamber in the 

 lower mammals does not extend beyond the point where 

 Descemet's membrane ends, a line drawn backward ver- 

 tically to the surface of the eyeball through the canal of 

 Schlemm does not pass through the angle of the anterior 

 chamber but some distance external to it, and the large 

 circular artery of the iris, together with a large portion of 

 the ciliary processes, lies to its inner side (Figs. 13 and 14). 



The concentration of the fibers of the ligamentum pectin- 

 atum together in the human eye forms a fixed point, or ten- 

 don of origin, for the ciliary muscle. In the lower mammals, 

 where the area occupied by the ligamentum pectinatum is 

 broader and more spaced out, the fibers of the ciliary muscle 

 do not have any fixed point of origin. Running longi- 

 tudinally they terminate anteriorly in the spaces between 

 the fibers of the ligamentum pectinatum, in the same way 

 as they terminate posteriorly between the fibers of the 

 lamina suprachoroidea (Fig. 14). 



A comparison of the ciliary muscle in its different degrees 

 of development in different mammals shows that the spindle- 

 shaped cells of which it is composed lie between the fibers 

 of the lamina suprachoroidea. 



In the human eye the fibers of the lamina, if traced for- 

 ward, can be seen to enter the posterior border of the ciliary 



