864 MOVEMENTS OF THE PUPIL. [BOOK m. 



usual vaso-constrictor impulses. A further support to this view 

 is afforded by the observations that the cervical sympathetic 

 does contain vaso-constrictor fibres for the blood vessels of the 

 iris, and that these leave the spinal cord by the same paths as 

 the pupil-dilator impulses, that is to say somewhat higher up 

 than the vaso-constrictor fibres for the ear. Nevertheless it 

 seems clear that the pupil-dilating influence exerted by the sym- 

 pathetic is something quite different from its vaso-constrictor 

 influence ; for the dilating effects of stimulating the sympathetic 

 may be witnessed in a bloodless eye, in which vaso-motor changes 

 could not produce their effect. Further, the changes in the 

 pupil and in the calibre of the iridic blood vessels are not coin- 

 cident; when the sympathetic is stimulated the widening of 

 the pupil begins some time before the constriction of the blood 

 vessels and indeed may, with a brief stimulation, be over and 

 past, before the latter has reached its maximum. Again, in 

 the extreme widening of the pupil which as we shall see is 

 brought about by atropin, and which seems to be of the same 

 nature as the widening caused by stimulation of the sympathetic, 

 the blood vessels of the iris need not be in the least constricted. 

 Moreover, it is stated that the long ciliary nerves which act as 

 pupil-dilators carry no vaso-constrictor impulses ; it is said that 

 stimulation of the long ciliary nerves while it widens the pupil 

 produces no vaso-motor effects, and after the division of the 

 long ciliary nerves stimulation of the cervical sympathetic, 

 while it produces vaso-constriction in the eye as in other parts 

 of the head and face, gives rise to no widening of the pupil. 

 The impulses along the fibres of the cervical sympathetic, which 

 cause widening of the pupil, must act in some manner other 

 than by giving rise to vascular changes. 



Did there exist in the eyes of animals an arrangement of 

 muscular fibres disposed radially from the circumference of the 

 iris to the pupil as conspicuous as the circular muscle, since 

 such a muscular arrangement would act as a dilator, there would 

 probably be a general agreement that the widening which results 

 from stimulation of the sympathetic is brought about by con- 

 traction of these dilator muscular fibres. But it is only in the 

 case of one or two kinds of animals that any such distinct radial 

 muscles are present in the iris, and even in these cases the 

 muscles are not conspicuous. In all other animals including 

 man, the only structure in the iris which can be appealed to as 

 a dilator muscle, is a peculiar nucleated layer at the hinder sur- 

 face, just in front of the pigment epithelium. The absence of 

 any clear indubitable dilator muscle has led some to explain the 

 pupil-dilating influence of the sympathetic as due to the impulses 

 along that nerve inhibiting the previously existing contraction 

 of the sphincter. These argue that the sphincter may be com- 

 pared to the heart, inasmuch as it possesses an automatic power 



