554 



PHYSIOLOGY 



therefore of the sympathetic nerve causes constriction, and section of the 

 third nerve dilatation of the pupil. These tonic influences are probably 

 reflex in origin, since it is found that, after cutting off afferent impressions 

 from 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 microscope, 

 the view has been put forward that dilatation of the pupil on stimulation of the sym- 

 pathetic nerve is due merely to the relaxation of the tonic contraction of the sphincter 



FIG. 279. Effect on iris of cat of local stimulation. 



The first effect, as in A, is to cause contraction of the constrictor pupil! se below 

 the electrodes, and this is succeeded in B by a strong localised contraction of the 

 radiating fibres. (LANGLEY and ANDERSON.) 



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. 279). In another experi- 

 ment they isolated a sector of the iris by two radial cuts ; on exciting this sector it 

 shortened, and the same effect was produced 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 hypertonus, 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. 



* Probably on account of the escape of adrenaline into the circulation, and its 

 sympatho-mimetric action on the iris. 



