68 SPECIAL SENSES. 



S. The sclerotic ; a dense, fibrous membrane, chiefly for 

 the protection of the more delicate structures of the globe, 

 and giving attachment to the muscles which move the eye- 

 ball. Attached to the sclerotic, are the tendons of R, the 

 recti muscles. 



Cor. The cornea ; a transparent structure, forming the 

 anterior, projecting sixth of the globe ; dense and resisting, 

 allowing, however, the passage of light ; covered, on its con- 

 vex surface, with several layers of transparent epithelial cells, 

 and, on its posterior surface, with the membrane of the aque- 

 ous humor. 



Cho. The choroid coat, lining the sclerotic and extending 

 only as far forward as the cornea ; connected with the scle- 

 rotic by loose connective tissue, in which ramify blood-vessels 

 and nerves, and presenting an external, vascular layer and an 

 internal, pigmentary layer, which latter gives its character- 

 istic dark-brown color. 



C. P. The ciliary processes ; peculiar folds of the choroid, 

 which form its anterior border, and which embrace the folds 

 of the suspensory ligament of the lens. 



C. M. The ciliary muscle, formerly called the ciliary liga- 

 ment ; a muscular ring, situated just outside of the ciliary 

 processes, arising from the circular line of junction of the 

 sclerotic with the cornea, and passing over the ciliary pro- 

 cesses, to become lost in the fibrous tissue of the choroid. 

 This is sometimes called the tensor of the choroid. Its action 

 is to tighten the choroid over the vitreous humor, relax the 

 ciliary processes and the suspensory ligament of the lens, 

 when the lens, by virtue of its elasticity, becomes more con- 

 vex. The ciliary muscle is the active agent in accommoda- 

 tion. In the figure, the mechanism of accommodation is 

 shown by the dotted lines, which represent the suspensory 

 ligament relaxed by the contraction of the ciliary muscle, and 

 the lens, increased in its convexity, pushing forward the iris. 



I. The iris ; dividing the space in front of the lens into 

 two chambers occupied by the aqueous humor : (A) The an- 



