498 THE SPECIAL SENSES. 



retina, at the same time that the object is removed to a greater dis- 

 tance from the eye, the focus of the convergent rays would still fall 

 upon the retina, and the image would still be distinct. In the op- 

 t posite case, where the object is brought nearer the eye, a similar 

 movement of the lens forward would again secure perfect vision. 

 Thus, when we look at near objects, the lens moves forward 

 toward the pupil ; when we look at remote objects, it moves back- 

 ward toward the retina. 



This movement of the lens is apparently accomplished by the 

 action of the ciliary muscle. This muscle (Fig. 155, e) arises, in 

 front, from the conjunction of the sclerotic and the cornea, and run- 

 ning backward and outward, is inserted into the anterior part of 

 the choroid, about the situation at which the hyaloid membrane 

 passes off, to become the suspensory ligament of the lens. As 

 already mentioned, this muscle is supplied with nervous filaments 

 from the ophthalmic ganglion. Its action is to draw the lens for- 

 ward, by means of its attachment to the hyaloid membrane and 

 choroid coat; and, in the human subject, the retreat or retrogres- 

 sion of the lens toward the retina, after the ciliary muscle is relaxed, 

 seems to be due to the elastic resiliency of the remaining tissues of 

 the eyeball. 



But in order to allow of such a backward and forward movement 

 of the lens, since the liquids of the eyeball are incompressible, there 

 must be a corresponding displacement of other parts, both before 

 and behind. This is undoubtedly provided for by the vascularity 

 of the choroid coat. This membrane is supplied with an exceedingly 

 abundant vascular plexus over its whole posterior portion ; and in 

 front it is thrown into a circle of prominent converging folds, or 

 processes, the ciliary processes, which are nothing more than erectile 

 congeries of bloodvessels, covered with the pigment of the choroid. 

 A portion of the ciliary processes projects in front of the lens, and 

 their vascular network is continued over a great part of the pos- 

 terior surface of the iris. Thus there is, both behind and in front 

 of the lens, an erectile system of bloodvessels ; and as these blood- 

 vessels become alternately empty or turgid, they will allow of the 

 displacement of the lens in an anterior or posterior direction. 



Accordingly there is a certain accommodation of the eye neces- 

 sary to the distinct sight of objects at different distances. But the 

 range of this accommodation is limited, and the same eye cannot be 

 made to see distinctly at all distances. For all ordinary eyes, the 

 accommodation fails, and vision becomes imperfect, when the object 



