754 



SIGHT. 



FIG. 172. 



tions of the nervous mechanism being intact, a contraction of the pupil (not 

 always very well marked) takes place, and if the peripheral portion (?'. e., the 



upper portion still connected with the 

 head) be stimulated, a well-developed 

 dilatation is the result. The sympa- 

 thetic has, it will be observed, an effect 

 on the iris the opposite of that which it 

 exercises on the bloodvessels ; when it 

 is stimulated the pupils are dilated 

 while the bloodvessels are constricted. 

 This dilating influence of the sympa- 

 thetic may, as in the case of the vaso- 

 motor action of the same nerve, be 

 traced back down the neck to the upper 

 thoracic ganglion and thence along the 

 raiui communicantes and roots of the 

 lower cervical and first dorsal or two 

 first dorsal spinal nerves, to a region 

 in the lower cervical and upper dorsal 

 cord (called by some authors the cen- 

 trum cilio-spinale inferius), and from 

 thence up through the medulla oblon- 

 gata to a centre, which appears to be 

 placed in the floor of the front part 

 of the aqueduct of Sylvius not far 

 from and apparently on either side 

 of the centre for contraction of the 

 pupil. 



634. The dilatation of the pupil 

 which is witnessed in dyspnoea, and 

 that which results from stimulation of 

 sensory nerves and from emotions, ap- 

 pears to be brought about by the 

 action of the sympathetic, the venous 

 blood or the sensory impulses or the 

 emotional impulses so affecting the 

 dilating centre as to augment the dilat- 

 ing impulses proceeding from it along 

 the sympathetic. The existence of the 

 subordinate centre in the cervical or 

 dorsal cord, spoken of just now, is sup- 

 posed to be indicated by the fact that 

 after division of the medulla oblongata, and consequent severance of the 

 efferent paths from the centre in the aqueduct of Sylvius, dilatation of the 

 pupil may still be brought about, in some animals at least, by dyspnoea or 

 by adequate stimulation of sensory nerves. A question is raised here in 

 fact somewhat similar to that raised in connection with the medullary respi- 

 ratory centre (p. 370) ; and here as there we may probably conclude that 

 the independent action of such a spinal centre is of subordinate importance. 

 635. The pupil then seems to be under the dominion of two antagon- 

 istic mechanisms : one a contracting mechanism, reflex in nature, the third 

 nerve serving as the efferent and the optic as the afferent tract ; the other a 

 dilating mechanism, apparently tonic in nature, but subject to augmentation 

 from various causes, and of this the cervical sympathetic is the efferent 

 channel. Hence, when the third or optic nerve is divided, not only does 



sym; 



Diagrammatic Representation of the 

 Nerves Governing the Pupil. II. optic 

 nerve ; I. y. lenticular ganglion ; r. b. its 

 short root from ///. ; oc. m.. third or oculo- 

 motor nerve ; sym. its sympathetic root ; r. I. 

 its long root from V. ophthm. the nasal branch 

 of the ophthalmic division of the fifth nerve , 

 s. c. the short ciliary nerves from the lenticu- 

 lar ganglion ; I. c. the long ciliary nerve from 

 the nasal branch of the ophthalmic division 

 of the fifth nerve. 



