142 HUMAN PHYSIOLOGY. 



in which the rays of light make no impression, owing to the absence of 

 the proper retinal elements ; the fibres of the optic nerve being insensible 

 to the action of light. 



The course which a ray of light takes is as follows : After passing 

 through the cornea, lens, and vitreous humor and the layers of the retina, 

 it is finally arrested by the pigmentary layer of the choroid ; here it excites 

 in the layer of rods and cones some physical or chemical change, which is 

 then transmitted to the fibres of the optic nerve, and thence to the brain, 

 where it is perceived as a sensation of light. 



The Accommodation of the eye to vision for different distances is ac- 

 complished by a change in the convexities of the lens, caused by the action 

 of the ciliary muscle. When the eye is accommodated for vision at far 

 distances, the structures are in a passive condition and the lens is flattened ; 

 when it is adjusted for vision at short distances, the convexities of the lens 

 are increased. 



When the Ciliary muscle contracts, the suspensory ligament is relaxed 

 and the lens becomes more convex, in virtue of its own elasticity. 



Optical Defects. Astigmatism is a condition of the eye which pre- 

 vents vertical and horizontal lines from being focused at the same time, and 

 is due to a greater curvature of the cornea in one meridian than another. 



Spherical aberration is a condition in which there is an indistinctness of 

 an image from the unequal refraction of the rays of light passing through 

 the circumference and the centre of the lens ; it is corrected mainly by the 

 iris, which cuts off the marginal rays, and only transmits those passing 

 through the centre. 



Chromatic aberration is a condition in which the image is surrounded 

 by a colored margin, from the decomposition of the rays of light into 

 their elementary parts. 



Myopia, or short-sightedness, is caused by an abnormal increase in the 

 antero- posterior diameter of the eyeball, or by a subnormal refracting 

 power of the lens ; it is generally due to the first cause ; the lens being too 

 far removed from the retina, forms the image in front of it, and the per- 

 ception becomes dim and blurred. Concave glasses correct this defect, by 

 preventing the rays from converging too soon. 



Hypermetropia, or long-sightedness, is caused by a shortening of the 

 antero-posterior diameter, or by an excessive refractive power of the lens ; 

 the focus of the rays of light would, therefore, be behind the retina. Con- 

 vex glasses correct this defect, by converging the rays of light more anteri- 

 orly. 



