866 MOVEMENTS OF THE PUPIL. [BOOK in. 



under the dominion of two antagonistic mechanisms : one a con- 

 stricting 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 sym- 

 pathetic is the efferent channel. Hence, when the third or optic 

 nerve is divided, not only do constricting impulses cease to be 

 manifest, but the effect of their absence is increased, 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 constriction results. 

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

 of the sympathetic is overcome, and constriction results ; and 

 when the sympathetic is stimulated, any constricting influence 

 of the third nerve which may be present is overcome, and dila- 

 tion ensues. 



The former, optic oculo-motor mechanism is the instrument 

 by means of which the pupil is adapted to the amount of light, 

 the latter, sympathetic mechanism appears to be employed when 

 other influences are brought to bear on the pupil. Thus the 

 characteristic pupil-dilating effects of emotions such as fear, of 

 the painful stimulation of sensory nerves, of dyspnrea, and in 

 part of some drugs, appear to be carried out through the sym- 

 pathetic mechanism. 



We may add that both these mechanisms may be thrown 

 into action by stimulation of certain parts of the ' ocular ' area 

 in the cerebral cortex ; constriction or dilation may be obtained 

 by stimulation of the appropriate spot. That the dilation which 

 is observed is brought about by means of the sympathetic mech- 

 anism, is shewn by the fact that it fails if the cervical sympa- 

 thetic be previously divided. 



542. In the case of many drugs, however, the effect pro- 

 duced is either in part or wholly independent of both these ner- 

 vous mechanisms. A small quantity of atropin introduced into 

 the system, or even directly into the eye, causes a dilation of the 

 pupil which may be so great that the iris is reduced to a mere 

 rim, while physostigmin (eserin) similarly introduced into the 

 system or eye produces a constriction of the pupil which may be 

 so great that the pupil is narrowed to a mere pin's point. Since 

 both these drugs may produce their full effects after division of 

 the optic oculo-motor and the sympathetic nerves, and indeed 

 may produce their effects in an extirpated eyeball, it is obvious 

 that those effects are not due to the drugs acting on the central 

 parts of the above mechanisms. Their action is a local one. 

 They do not act by means of the ciliary ganglion, for both drugs 

 continue to produce their effects to a most marked degree after 

 the ganglion has been excised. Nor have we any evidence 

 that their action is dependent on any other local nervous mech- 



