DIOPTRIC MECHANISMS OF THE EYEBALL 



617 



the retina by division of the optic nerve, section of the third nerve 

 produces no further dilatation of the pupil. 



Since the dilatator muscle is often difficult to demonstrate under the micro- 

 scope, the view has been put forward that dilatation of the pupil on stimulation 

 of the sympathetic nerve is due merely to the relaxation of the tonic contraction 

 of the sphincter pupillse. The following experiments by Langley and Anderson 

 showed definitely the erroneousness of this view: 



On stimulating the corneo -sclerotic junction so as to excite a limited portion 

 of the iris, a well-defined local dilatation of the pupil is produced. If the 

 dilatation were due to the relaxation of the sphincter, the dilatation could not 

 be local, but would have extended to the whole circumference of the pupil 



FIG. 277. Effect on iris of cat of local stimulation. 

 The first effect, as in A, is to cause contraction of the constrictor pupillee 

 below the electrodes, and this is succeeded in B by a strong localised con- 

 traction of the radiating fibres. (LANGLEY and ANDERSON.) 



(Fig. 277). In another experiment they isolated a sector of the iris by two 

 radial cuts ; on exciting this sector it shortened, and the same effect was pro- 

 duced by excitation of the sympathetic in the neck, although any action 

 of the sphincter must have been abolished by the mode of preparation. 

 Section of the sympathetic in the neck causes lasting constriction of the 

 pupil, and the same effect is produced by extirpation of the superior cervical 

 ganglion. After the lapse of some time, however, the muscles, freed from their 

 nervous connections with the ganglion, enter easily into a condition of hyper- 

 tonus, so that the pupil on the side of the lesion may be more dilated than on 

 the normal side. This hypertonus is especially marked when a slight amount 

 of asphyxia or rise of blood pressure is present. 



THE OPHTHALMOSCOPE 



By means of this instrument we are enabled to obtain a magnified picture 

 of the back of the eyeball, as well as to judge of the presence and degree of 

 abnormalities in the refracting apparatus of the observed eye. 



When light falls on the eye through the pupil the greater part of it is absorbed 



