400 THE ORGANS OF SPECIAL SENSE 



and smooth externally, excepting where it receives 

 the insertion of the recti and obliqui muscles. It 

 covers the posterior five-sixths of the eye-ball. Behind 

 it receives the optic nerve at a point just internal to 

 the centre, the fibrous sheath of the former being 

 continuous with the sclerotic. 



The cornea forms the anterior sixth of the external 

 coat. It is transparent and projecting, and nearly 

 an arc of a true sphere, the anterior surface being 

 convex and the posterior surface concave. 



The choroid or intermediate coat is continued into the 

 cornea. It is a chocolate-colored, vascular structure 

 lying between the sclerotic and retina and investing 

 the posterior five-sixths of the eye-ball, blending in front 

 with the iris after forming a number of folds, the 

 ciliary processes. 



The ciliary muscle is a circular plane of unstriped 

 muscle placed between the choroid and sclerotic at 

 its anterior part. It consists of circular and radiating 

 fibers. This muscle aids in contracting and dilating 

 the size of the pupil in response to nerve stimuli, and 

 under normal conditions regulates the amount of 

 light entering the chamber of the eye. 



The iris gives to the eye its color (depending on 

 the pigment present). It is a thin, contractile, circu- 

 lar membrane presenting, at about its centre, a cir- 

 cular aperture, the pupil. It is suspended in the 

 aqueous humor behind the cornea and in front of 

 the lens. 



The arteries are supplied from the long and anterior 

 ciliary. The nerves are branches of the lenticular 

 ganglion and the long ciliary from the nasal branch 

 of the ophthalmic. They form a plexus around the 

 circumference of the iris, and end in the muscular 

 fibers and in a network on the front of the iris. The 

 nerves to the circular fibers come from the motor 

 oculi; those to the radiating, from the sympa- 

 thetic. 



