CHAP. IT.] SIGHT. 503 



head) be stimulated, a well-developed dilation is the result. The 

 sympathetic has, it will be observed, an effect on the iris, the 

 opposite of that which it exercises on the blood-vessels ; when it is 

 stimulated the pupils are dilated while the blood-vessels are con- 

 stricted. This dilating influence of the sympathetic 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 

 rami 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 centrum cilio- 

 spinale inferius), and from thence up through the medulla oblongata 

 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. 



The dilation of the pupil which is witnessed in dyspnrea, and that 

 which results from stimulation of sensory nerves and from emotions, 

 appears 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 dilating impulses 

 proceeding from it along the sympathetic. The existence of the 

 subordinate centre in the cervical or dorsal cord, spoken of just 

 now, is supposed 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, dilation 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 respiratory centre (p. 355) ; and 

 here as there we may probably conclude that the independent 

 action of such a spinal centre is of subordinate importance. 



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

 nistic 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 contraction of the 

 pupil cease to be manifest, but active dilation occurs, on account of 

 the tonic dilating influence of the sympathetic being left free to 

 work. When, on the other hand, the sympathetic is divided, this 

 tonic dilating influence falls away, and contraction results. When 

 the optic or third nerve is stimulated, the dilating effect of the 

 sympathetic is overcome, and contraction results ; and when the 

 sympathetic is stimulated, any contracting influence of the third 

 nerve which may be present is overcome, and dilation ensues. 



But there are considerations which shew that the matter is still 

 more complex than this. A small quantity of atropin introduced 

 into the eye or into the system causes a dilation of the pupil. This 



