570 ANIMAL PHYSIOLOGY. 
now briefly detail: 1. When the third nerve is divided, stimu- 
lation of the optic nerve (or retina) does not cause contraction 
of the pupil as usual. 2. When the optic nerve is divided, light 
no longer causes a contraction of the pupil, though stimulation 
of the third nerve or its center in the anterior portion of the 
floor of the aqueduct of Sylvius does bring about this result. 
3. Section of the cervical sympathetic is followed by contrac- 
tion and stimulation of its peripheral end by dilation of the 
pupil. 
From such experiments it has been concluded that—1. The 
optic is the afferent nerve and the third nerve the efferent nerve 
concerned in the contraction of the pupil; and that the center 
in the brain is situated as indicated above, so that the act is or- 
dinarily a reflex. 2. That the cervical sympathetic is the path 
of the efferent impulses regulating the action of the radiating 
fibers of the iris. 
Its center has been located near that for the contraction of 
the pupil, and it may be assumed to exert a tonic action over 
the iris comparable to that of the vaso-motor center over the 
blood-vessels. 
The impulses may be traced through the cervical sympa- 
thetic and its ganglia back to the first thoracic ganglion, and 
thence to the spinal cord and brain. There may be subsidiary 
centers in the cervical spinal cord. 
There are facts which it is difficult to explain in the above 
manner. Thus, when atropin is dropped into the eye, the dila- 
tation is greater than that which follows section of the optic 
nerve or the third nerve. In such a case, paralysis of the con- 
tracting mechanism, by which the dilating mechanism is left 
free to act, should produce, we might suppose, the greatest pos- 
sible dilation of the pupil, especially if we assume, as some do, 
that there are no radiating muscular fibers, but that all the 
effects are produced through the sphincter of the iris; but such 
is not the case. The result has been set down to the action of 
the drug upon a local nervous mechanism, or the muscular 
fibers themselves, or to the vaso-motor changes said to be co- 
incident. This view is strengthened by the fact that stimu- 
lation of the retina in a recently removed eye will cause some 
reflex contraction of the pupil. In explaining the action of 
drugs on the pupil we are not limited to either a purely local 
or a purely central influence; some seem to act in one stage 
more upon the centers, in another more locally. Vaso-motor 
influences undoubtedly do affect the size of the pupil, full vessels 
