324 AN AMERICAN TENT-BOOK OE PHYSIOLOGY. 



plete crossing of the optic nerves, and consequently a unilateral pupillary 

 reflex. 1 



A direct as well as a reflex constriction of the pupil under the influence of 

 light has been observed in the excised eyes of eels, frogs, and some other ani- 

 mals. As the phenomenon can be seen in preparations consisting of the iris 

 alone or of the iris and cornea together, it is evident that the light exerts its 

 influence directly upon the tissues of the iris and not through an intraocular 

 connection with the retina. The maximum effect is produced by the yellowish- 

 green portion of the spectrum. 



Antagonizing the motor oculi nerve in its constricting influence on the 

 pupil is a set of nerve-fibres the function of which is to increase the size of 

 the pupil. Most of these fibres seem to run their course from a centre which 

 lies in the floor of the third ventricle not far from the origin of the third pair, 

 through the bulb, the cervical cord, the anterior roots of the upper dorsal 

 nerves, the upper thoracic ganglion, the cervical sympathetic nerve as far as 

 the upper cervical ganglion ; then through a branch which accompanies the 

 internal carotid artery, passes over the Gasserian ganglion and joins the oph- 

 thalmic branch of the fifth pair ; then through the nasal branch of the latter 

 nerve and the long ciliary nerves to the eye 2 (see diagram, p. 323). These 

 fibres appear to be in a state of tonic activity, for section of them in any part 

 of their course (most conveniently in the cervical sympathetic) causes a con- 

 traction of the pupil which, on stimulation of the peripheral end of the divided 

 nerve, gives place to a marked dilatation. Their activity can be increased in 

 various ways. Thus dilatation of the pupil may be caused by dyspnea, vio- 

 lent muscular efforts, etc. Stimulation of various sensory nerves may also 

 cause reflex dilatation of the pupil, and since this phenomenon may be observed, 

 though greatly diminished in intensity, after extirpation of the superior cervi- 

 cal sympathetic ganglion, it is probable that the dilatation depends in part 

 upon a reflex inhibition of the constrictor nerves. 



Since the cervical sympathetic nerve contains vaso-constrictor fibres for the 

 head and neck, it has been thought that its dilating effect upon the pupil might 

 be explained by its power of causing changes in the amount of blood in the 

 vessels of the iris. There is no doubt that a condition of vascular turgescence 

 or depletion will tend to produce contraction or dilatation of the pupil, but it is 

 impossible to explain the observed phenomena in this way, since the pupillary 

 are more prompt than the vascular changes, and may be observed on a bloodless 

 eye. Moreover, the nerve-fibres producing them are said to have a somewhat 

 different course. Another explanation of the influence of the sympathetic on 

 the pupil is that it acts by inhibiting the contraction of the sphincter muscles, 

 and that the dilatation is simply an elastic reaction. But since it is posssible to 

 produce local dilatation of the pupil by circumscribed stimulation at or near 



1 Steinach : Archiv fur die gesammte Physiologic, xlvii. 313. 



2 Langley : Journal of Physiology, xiii. p. 575. For the evidence of the existence of a 

 "cilio-spinal " centre in the cord, see Steil and Langendorff : Archiv far die gesammte Phys- 

 iologic, lviii. S. 155; also Sclienck : Ibid., Ixii. S. 19 L 



