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It is much more difficult to decide in respect to the nature of the 
trigeminal component. We have as a premise for our reasoning the 
observations of Zeglinski (’85) to the effect that direct stimulation of 
the. trigeminal nerve consistently causes dilatation of the pupil. This 
reaction is obtained in mammals by excitation of the cervical sympathetic. 
In birds, however, such stimulation produces either no effect on the 
iris (Zeglinski, ’85, Jegorow, ’87) or at most one which is slight 
and inconstant (Langley, :03*). Zeglinski is convinced that in birds 
all the pupil-dilator fibers, whatever their origin may be, reach the eye 
through the ramus ophthalmicus trigemini. 
What, then, is the probable source of these fibers? Are they sym- 
pathetic or do they belong to the motor root of the trigeminus? Micros- 
copical examination fails to furnish the answer, since in the hen sym- 
pathetic fibers cannot be distinguished from fine cerebro-spinal fibers, 
the former as well as the latter being provided with medullary sheaths. 
If the pupil-dilator fibers of the ophthalmicus are sympathetic in origin 
it is difficult to understand why stimulation of the cervical sympathetic 
trunk, with which they should be connected, fails to produce an effect 
on the iris. On the developmental side it has been shown (Carpenter, 
:06) that in chick embryos the fibers of the ramus communicans grow 
toward and connect directly with the anlage of the ciliary ganglion 
during the fifth day of incubation. At this time, at least, the ophthalmic 
division of the trigeminus shows no sympathetic connections. 
The evidence at our command seems to be in favor of our regard- 
ing the pupil-dilator fibers running in the ophthalmicus as cerebro- 
spinal neurones belonging to the motor component of the trigeminal 
nerve. Can we follow the impulse which passes along them to the dilator 
muscle of the iris? Does it travel by the long ciliary nerves given off 
by the ramus communicans, or does it pass into the ciliary ganglion 
via the radix longa, and thence along short ciliary fibers to the eyeball? 
Here we are driven to reasoning by analogy, since we possess no data 
bearing directly on this point. We have seen that the pupil-constrictor 
system is made up of two sets of neurones, preganglionic (oculomotor) 
and postganglionic (short ciliary). There may be some requirement of 
this pupillary reflex mechanism which demands, along the motor limb 
of the arc, a second, peripheral neurone connected with the end organ. 
These conditions are met by the radix longa and the short ciliary fibers 
continuous with it, while they are not met by the long ciliary nerves. 
We are perhaps warranted, therefore, in looking upon the path through 
the cilary ganglion via the radix longa as the one taken by the 
efferent impulses in the reflex causing dilatation of the pupil. This 
assumption that the fibers of the long root are motor leaves the long 
