578 MANUAL OF PHYSIOLOGY. 



produced by stimulating the retina, even when the eye is cut off 

 from the brain centres, and (2) that the local effect of atropia 

 in dilating, and calabar bean in narrowing the pupil, seem in a 

 measure independent of the central nerve organs, it has been 

 concluded that there must be some local nerve mechanism in the 

 eye itself capable of reflecting nerve stimuli and being affected 

 by these poisons. 



The student must never lose sight of the circumstances under 

 which the pupils contract, namely : 



1. The application of strong light, even to one retina, causes 

 reflex stimulation of the ciliary nerves. 



2. Stimulation of the nasal and ophthalmic branches of the 

 fifth afferent nerve excites the sphincter. 



3. Contraction of the pupil is " associated " with accommoda- 

 tion for near objects. 



4. Similar "associated "contraction accompanies inward move- 

 ment of the eyeball. 



5. During sleep, or as the result of vasomotor disturbances in 

 the brain (ansemia), the pupil contracts. 



6. Under the influence of physostigmatin, nicotin and morphia. 



7. From any stimulation of the optic or third nerves or the 

 corpora quadrigemina. 



The circumstances in which the pupils are found to be dilated 

 are equally important from a practical point of view, namely : 



1. In the dark or with insensitive retina3. 



2. Irritation of the cervical sympathetic. 



3. Under the influence of atropin, daturin, etc. 



4. In asphyxia or dyspnoea from venosity of the blood. 



5. Painful sensations from the skin, etc. 



THE OPHTHALMOSCOPE. 



When we look into the eye the pupil appears quite black, 

 because the illumination of the eye chamber is not sufficient to 

 show its parts from the outside, when the light is strong. In the 

 same way when we try to look into a room in the daytime 

 through the window, we see nothing in the depth of the room, 

 because the light outside is so much stronger than that within 



