576 MANUAL OF PHYSIOLOGY. 



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 frame-work of delicate connective tissue, 

 like that of the choroid coat, containing many blood vessels. On 

 its posterior surface is a dense layer of pigment cells, called he 

 uvea, which gives the eye its color. The motions of contracting 

 and dilating the pupil are carried out by smooth muscle fibres. 

 The act of contracting the pupil is performed by a very definite 



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 

 toward the choroid to the right, where it separates the latter from the sclerotic. 



set of fibres forming the sphincter, which surrounds the margin 

 of the pupil, while other fibres are said to radiate from the pupil 

 to the attached margin of the iris. The sphincter muscle seems 

 always to be more or less in action, because, if it be paralyzed, 

 the action of the dilating forces becomes obvious. But the mus- 

 cular character of the dilator has been doubted, from the fact 

 that the fibres have not been satisfactorily demonstrated. Cer- 

 tainly the sphincter seems to be the stronger of the two, for strong 

 electric stimulation causes contraction of the pupil, and shortly 

 after death the pupils dilate. We must assume that the power 

 of the sphincter dies more quickly than that of the dilator, or 



