THE EYE 1535 



Derivatives ol the Mesodermic Envelope of the Optle Cap. — These are as 



follows: (i) Sclerotic, (2) cornea, {3) choroid, {4) ciliary body (including the 

 ciliary processes and ciliary muscle), and (5) iris. 



The mesoderm which invests the outer surface ol the optic cnp is disposed 

 in two layers — outer and inner. The outer layer has a fibrous character, and 

 gives rise to the sclerotic, of which the eornea is a for\vard extension. The 

 inner layer is vascular, cind gives rise to the choroid, ciliary body, and iris. 

 The outer dense fibrous layer of the mesoderm of the outer surface of the 

 optic cup, as stated, gives rise to the sclerotic. From its anterior margin a 

 thick lamina of mesoderm is prolonged between the lens-vesicle and the surface 

 ectoderm. This lamina di\-ides into two layers — superficial and deep. The 

 superficial layer becomes differentiated into the eomea, which is thus genetic- 

 ally continuous with the sclerotic. The deep layer becomes the anterior wall 

 of the lens-capsule. Between these two layers there is an interval, which 

 represents the aqueous chamber, this chamber being subsequently divided 

 by the iris into the anterior and posterior aqueous chambers. 



The inner vascular layer of the mesoderm of the outer surface of the optic 

 cap, as stated, gives rise to the choroid. The anterior margin ol the choroid, 

 which adjoins the margin of the optic cup (ciliar}' region) becomes thickened, 

 and gives rise to the ciliary body, in connection with which the ciliary processes 

 and ciliary muscle are developed. The ciliary processes become covered 

 posteriorly by the pars ciliaris refines, which is a thin retinal expansion from 

 the ciliary region of the optic cup. The iris is also developed at the anterior 

 margin of the choroid in the form of a ring of mesoderm. In this mesoderm 

 the fibres forming the dilator pupillce and sphincter pupUlcB muscles are formed, 

 and the back of the iris receives a pigmentary covering (uvea) from the pars 

 iridica retina, which is a thin retinal extension from the ciliary region of the 

 optic cup. 



