VISION. 571 - 
tending to contraction and the reverse to dilation. Upon the 
whole, it seems best to regard the two mechanisms as supple- 
mentary to one another, so that usually with increased action 
of the one there is diminished action of the other. We find 
that the two eyes move in harmony, and that the two pupils in 
health are always of the same size. Light thrown upon one 
eye contracts the pupil of the other.. We are thus led to be- 
lieve in associated or consensual movement of the iris, owing 
to nervous connections between the various centers involved. 
These are physiological, but whether anatomical or not, in the 
sense that annectant fibers exist, is uncertain; and, however, 
in the evolution of function, they may have been at first pro- ' 
duced, have been so strengthened, according to the law of habit, 
that now it is with the greatest difficulty that one may learn 
to move one eye independently of the other, or modify the 
form of the pupils without also shifting the visual axes. 
It is to be remembered that, although the dilating center is 
automatic in action, it may also act reflexly, or be modified by 
unusual afferent impulses—as, e. g., the strong stimulation of 
any sensory nerve which causes enlargement of the pupil 
through inhibition of the center. To render the paths of 
impulses affecting the iris somewhat clearer, it is well to bear 
in mind the nervous supply of the part: 1. The third nerve, 
through the ciliary (ophthalmic, lenticular) ganglion, supplies 
short ciliary nerves to the iris, ciliary muscle, and choroid. 2. 
The cervical sympathetic reaches the iris chiefly through the 
long ciliary nerves and the ophthalmic division of the fifth. 
3. There are sensory fibers from the fifth nerve; and, according 
to some observers, also dilating fibers from this nerve inde- 
pendent of the sympathetic, as well as those that may reach 
the eye by the long ciliary nerves without entering the ciliary 
ganglion. 4, The centers from which both the contracting and 
dilating impulses proceed are situated near to each other in 
the floor of the aqueduct of Sylvius. It is of practical im- 
portance to remember the various circumstances under which 
the pupil contracts and dilates. 
Contraction (Myosis).—1. Access of strong light to the 
retina, 2. Associated contraction on accommodation for near 
objects. 3. Similar associated contraction when the visual axes 
converge, as in accommodation for near objects. 4. Reflex 
stimulation of afferent nerves, as the nasal or ophthalmic divis- 
ion of the fifth nerve. 5. During sleep. 6. Upon stimulation 
of the optic or the third nerve, and the corpora quadrigemina 
