862 MOVEMENTS OF THE PUPIL. [BOOK in. 



Hence, when a change in the pupil of one eye is brought about 

 by some means other than the one we are now considering, the 

 pupil of the other eye is affected ; when for instance one pupil 

 is dilated with atropin, the larger amount of light thus admitted 

 into that eye causes a narrowing of the pupil of the other eye, 

 and thus increases the difference between the pupils of the two 

 eyes. In the lower mammals and other vertebrates, the mech- 

 anisms in question are independent, stimulation of one retina 

 produces no effect on the pupil of the other eye. 



It is by means of this reflex mechanism of which we have 

 just given a sketch that the changes of the pupil which take 

 place in actual life are to a large extent carried out; a con- 

 stricted pupil indicates in the majority of instances an activity 

 of the reflex mechanism, and a dilated pupil the absence of or 

 diminution of that activity. In the normal, healthy organism 

 the activity of the mechanism is in the first instance dependent 

 on the amount of light falling on the retina ; but even in the 

 normal condition, and still more in an abnormal condition of the 

 organism, other influences may become dominant. The activity 

 of the centre may be exalted or depressed by nervous or other 

 actions ; the retina or optic nerves may be affected by the same 

 amount of light to a degree less than or greater than the normal, 

 and the efferent limb of the chain may be less or more effective. 



540. Besides, however, all the various changes which may 

 thus be induced by playing upon the optic-oculo-motor reflex 

 mechanism, other agencies are able to act on the pupil quite apart 

 from this reflex mechanism ; some of these act through the second 

 mechanism of which we spoke, and to which we can now turn. 



If the cervical sympathetic in the neck be divided, all other 

 influences which could possibly affect the pupil being avoided, a 

 constriction of the pupil will be seen to take place ; this however 

 is at times (in animals) not very well marked ; but, whether it 

 be so or no, if the peripheral portion of the nerve (i.e. the upper 

 portion still connected with the head) be stimulated, a well-de- 

 veloped dilation is the result. The cervical sympathetic has, it 

 will be observed, an effect on the pupil, the opposite of that which 

 it exercises on the blood vessels of the head and neck ; when it 

 is divided, the pupil becomes constricted but the blood vessels 

 dilate, and when it is stimulated the pupil is dilated while the 

 blood vessels are constricted. This pupil-dilating influence of 

 the cervical sympathetic may, as in the case of the vaso-con- 

 strictor action of the same nerve, be traced backwards down the 

 neck to the upper thoracic ganglion, and thence to the spinal 

 cord along the rami communicantes and anterior roots of certain 

 thoracic nerves. In all the higher animals the chief channel is 

 the second thoracic nerve ; in the cat, dog, monkey, and probably 

 in man some of the impulses pass by the first thoracic nerve and 

 a few by the third ; in the rabbit a very few pass by the first 



