574 



.MANUAL OF PHYSIOLOGY. 



In using the microscope they often annoy the unpracticed ob- 

 server. 



THE IRIS. 



It has already been mentioned that the motions of the iris alter 

 the size of the pupillary opening through which the rays of light 

 must pass, and while it regulates the amount of light admitted, 

 it always cuts off a large amount of the marginal rays, acting 

 like the diaphragm of an optical instrument. The great import- 



r 



FIG. 227. 



Section through the ciliary region, showing the relation of the iris (/) to 

 the choroid and the ciliary muscle (a), which arises from the margin of the 

 cornea at (e), and passes towards the choroid to the right, where it separates 

 the latter from the sclerotic. 



ance of not allowing the rays which would traverse the margin 

 of the lens to enter the eyeball can be understood after what has 

 been said of spherical aberration. But the iris also moves so as 

 to contract the pupil when the eye is adjusted for near vision, 

 independently of the intensity of the light by which the object 

 is illuminated. This action is of great advantage in viewing 

 near objects, because the more convex the lens becomes, the more 

 injurious are the marginal rays. If the iris did not thus contract 

 in near vision, the nearer we brought an object to our eye the 

 greater would be the tendency to indistinctness caused by spher- 

 ical aberration. 



The iris consists of a framework of delicate connective tissue, 

 like that of the choroid coat, containing many bloodvessels. On 



