3 I2 



THE SENSES 



front it is connected with the iris. The choroid is very vas- 

 cular. Its color is dark brown on account of the abundance 

 of pigment in the cells on the inner surface of the mem- 

 brane. Anteriorly the choroid is folded in upon itself to 

 form the ciliary processes, which project inward around the 

 margin of the crystalline lens. 



The ciliary muscle is important in accommodation. It is 



FIG. 89. Diagram of a vertical section of the eye. (From Yeo 

 after H olden.} 



i, anterior chamber filled with aqueous humor; 2, posterior chamber; 3, 

 canal of Petit; a, hvaloid membrane; b, retina (dotted line); c, choroid coat 

 (black line); d, sclerotic coat; e, cornea; f, iris; g, ciliary processes; h, canal of 

 Schlemm or Fontana; i, ciliary muscle. 



in the shape of a muscular ring surrounding the margin of 

 the choroid just outside the ciliary processes. In front it is 

 attached to the line of junction of the cornea and sclerotic 

 and to the ligament on the anterior surface of the iris ; be- 

 hind it is lost in the substance of the choroid. Its contrac- 

 tion, therefore, compresses the vitreous humor and relaxes 

 the suspensory ligament of the lens. The iris is a circular 

 veil hanging in front of the lens. It presents a perforation a 



