59G HANDBOOK OF PHYSIOLOGY. 



branches of that nerve, but by the short ciliary branches from the oph- 

 thalmic ganglion. 



The close sympathy subsisting between the two eyes is nowhere better 

 shown than by the condition of the pupil. If one eye be shaded by the 

 hand its pupil will of course dilate; but the pupil of the other eye will 

 also dilate, though it is unshaded. 



Ciliary Muscle. The ciliary muscle is composed of plain muscular 

 fibres, which form a narrow zone around the interior of the eyeball, near 

 the line of junction of the cornea with the sclerotic, and just behind the 

 outer border of the iris. The outermost fibres of this muscle are at- 

 tached in front to the inner part of the sclerotic and cornea at their line 

 of junction, and diverging somewhat, are fixed to the ciliary processes, 

 and a small portion of the choroid immediately behind them. The 

 inner fibres immediately within the preceding, form a circular zone 

 around the interior of the eyeball, outside the ciliary processes. They 

 compose the ring formerly called the ciliary ligament. 



ACCOMMODATION. 



The distinctness of the image formed upon the retina, is mainly de- 

 pendent on the rays emitted by each luminous point of the object being 

 brought to a perfect focus upon the retina. If this focus occur at a 

 point either in front of, or behind the retina, indistinctness of vision 

 ensues, with the production of a halo. The focal distance, i. e., the dis- 

 tance of the point at which the luminous rays from a lens are collected, 

 besides being regulated by the degree of convexity and density of the 

 lens, varies with the distance of the object from the lens, being greater 

 as this is shorter, and vice versa. Hence, since objects placed at various 

 distances from the eye can. within a certain range, different in different 

 persons, be seen with almost equal distinctness, there must be some pro- 

 vision by which the eye is enabled to adapt itself, so that whatever length 

 the focal distance may be, the focal point may always fall exactly upon 

 the retina. 



This power of adaptation of the eye to vision at different distances has 

 received the most varied explanations. It is obvious that the effect 

 might be produced in either of two ways, viz., by altering the convexity 

 and thus the refracting power, either of the cornea or lens; or by chang- 

 ing the position either of the retina or of the lens, so that whether the 

 object viewed be near or distant, and the focal distance thus increased 

 or diminished, the focal points to which the rays are converged by the 

 lens may always be" at the place occupied by the retina. The amount of 

 either of these changes required in even the widest range of vision, is 

 extremely small. For, from the refractive powers of the media of the 

 eye, the difference between the focal distances of the images of an object 

 at such a distance that the rays are parallel, and of one at the distance 



