VISION. 287 



meridians. This causes the rays of light in one plane to be 

 brought to a focus before those in other planes, so that the two 

 hands of a clock, when they are at right angles to each other, 

 cannot be seen distinctly at the same time, although they can be 

 successively focused. A certain amount of astigmatism exists 

 in every eye, but when it becomes extreme, it is necessary to 

 correct it by prescribing glasses which are astigmatic in the 

 opposite meridian to that of the eye. Such glasses are called 

 cylindrical. 



Astigmatism may occur along with either myopia or hyper- 

 metropia, and when any of these errors is only slight in degree, 

 the patient may be able, by efforts of accommodation, to over- 

 come the defect. The strain thus thrown on the ciliary muscle 

 is, however, quite commonly the cause of severe headache. The 

 correction of the errors should never be left to untrained per- 

 sons, but a proper oculist should be consulted, since it is usually 

 necessary to give atropin so that the accommodation may be 

 paralyzed and the exact extent of the error measured. The use 

 of improper glasses may aggravate the defect of vision and do 

 much more harm than good. 



The Sensory Apparatus of the Eye. 



The Functions of the Retina. The image which is formed on 

 the retina by the optical system of the eye sets up nerve im- 

 pulses which travel by the optic nerve to the visual center in 

 the occipital lobes of the cerebrum (see p. 272), where they are 

 interpreted. Microscopic examination of the retina has shown 

 that it consists of several layers of structures, the innermost 

 being of fine nerve fibers which arise from an adjacent layer of 

 large nerve cells, and the outermost of peculiar rod or cone- 

 shaped cells, called the rods and cones. Between the layer of 

 large cells and the layer of rods and cones are several layers 

 composed of other nerve cells and of interlacements of the pro- 

 cesses of cells and nerve fibers. The rods and cones are the 

 structures acted on by light, the other layers of the retina being 

 for the purpose of connecting the rods and cones with the large 

 nerve cells from which the fibers of the innermost laver arise. 



