THE EYE AND THE SENSE OF SIGHT 



107 



in radiating folds called the ciliary processes, which are 

 also covered by the pigment. 



143. The iris is a ring of plain, or involuntary, mus- 

 cular tissue. Its circular fibers by contracting narrow 

 the pupil, while its radi- 

 ating muscular or elastic 



fibers by their contrac- 

 tion dilate the pupil 

 when the circular muscle 

 is relaxed. Fibers from 

 the third cranial nerve 

 are distributed to the 

 circular muscle, and oth- 

 ers from the sympa- 

 thetic nervous system 

 are also found in this 

 muscle of the iris. The 

 pigment of the iris gives 

 what we call the " color 

 of the eye." The pupil 

 is simply an opening 

 through the iris into the 

 dark chamber beyond. 



144. The Retina. The 

 third of the coats of the 

 eye is the retina. This 

 is composed essentially 

 of the end fibers of the 

 optic nerve and nerve 



cells, which, with a supporting skeleton of connective tis- 

 sues, form a thin membrane lying loosely upon the choroid 

 and covering it as far as the ciliary processes. The retina 

 is too complex to admit of full description here. Though 



Fig. 64. Section of the retina. 



A diagram of the structure of the retina 

 as seen with the compound microscope. 



B the essential nervous elements of the 

 retina as demonstrated by the Golgi 

 method. 



1 internal limiting membrane. 



2 nerve-fiber layer. 



3 nerve-cell or ganglion-cell layer.- 



4 inner molecular layer. 



5 inner granular layer. 



6 outer molecular layer. 



7 outer granular layer. 



8 external limiting membrane. 



9 rod-and-cone layer. 

 10 pigment-cell layer. 



