760 



ACTION OF DRUGS ON THE PUPIL. 



chamber, because, owing to the diminished intraocular pressure, there is less resistance to the 

 passage of blood into the blood-vessels of the iris (Henscn and Vblckers) ; paralysis of the vaso- 

 motor fibres of the iris fl 347, 2). Conversely, the pupil is dilated by conditions the reverse of 

 those already mentioned, and also by strong muscular exertion, whereby blood flows freely into 

 the dilated muscular blood-vessels ; also, when death takes place. The condition of the filling 

 of the blood- vessels also explains the fact, that the pupil dilated with atropin becomes smaller 

 when a part of the sympathetic in the upper cervical ganglion, carrying the vaso-motor fibres 

 of the iris, is excised ; also, that after extirpation of this ganglion, atropin always causes a less 

 diminution of the pupil on this side. The fact that when the pupil is already dilated by 

 stimulation of the sympathetic, it is further dilated by atropin, is due to a diminished injection 

 of the blood-vessels of the iris. If an animal with its pupils dilated with atropin be rapidly 

 Med, the pupils contract, owing to the anemic stimulation of the origin of the oculomotorius 

 {Moriggia). The dilatation of the pupils observed in cases of neuralgia of the trigeminus, is 

 partly due to the stimulation of the dilating fibres, partly to the stimulation of the vaso-motor 

 fibres of the iris ( 347, 2). 



5. Contraction of the pupil occurs as an associated movement, during accommodation for a 

 near object (p. 752, 5), and when the eyeballs arc rotated inwards, which is the case during 

 .sleep (p. 685). Conversely, intense movements of the iris, caused by variations in the bright- 

 ness or dazzling illumination, e.g., of the electric light, are followed by disturbing associated 

 movements of the ciliary muscle (Ljubinsky). In certain movements discharged from the 

 medulla oblongata (forced respiration, chewing, swallowing, vomiting), dilatation of the pupil 

 occurs as a kind of associated movement. 



[Argyll Robertson Pupil. In this condition the pupil does not contract to light, although 

 it contracts when the eye is accommodated for a near object, vision usually being normal. The 

 ^ lesion is situated in those structures connecting 



the afferent and efferent fibres at their central 

 ends (at A in fig. 544), i.e., the connection 

 between the corpora quadrigemina and the oculo- 

 motorius. It is most frequently found in loco- 

 motor ataxia, although it also occurs in progres- 

 sive paralysis of the insane.] 



Direct stimulation at the margin of the cornea 

 causes dilatation of the pupil (E. H. Weber) ; in 

 fact, direct stimulation of circumscribed areas of 

 the margin of the iris causes partial contraction 

 of the dilator fibres (Bernstein and Dogiel). 

 Stimulation near the centre of the cornea con- 

 tracts the pupil (B. H. Weber). In addition, 

 we must assume that the iris itself contains ele- 

 ments that influence the diameter of the pupil 

 (Sig. Mayer and Pribram). 



Our knowledge of the action of poisons on the 



iris is still very obscure. Those substances which 



dilate the pupil are called mydriatics, e.g., 



,M atropin, homatropin, duboisin, daturin, and hyo- 



scyamin. They act chiefly by paralysing the 



oculomotorius. But, in addition, there must be 



** also an effect upon the dilating fibres, for after 



complete paralysis (section) of the oculomotorius, 



Fig 544 the moderate dilatation of the pupil thereby 



Scheme of the nerves of the iris. B, centrum l d " c n ed 345 ' ?>* ati[ } furth . er increased by 



optici; C, oculomotor centre: D, dilator 2T\ JS^f ^ P f n T n 



centre (spinal); E, iris ; G, optic nerve ; HguJTft stimulatl n of the P l \ pi11 ?' 



H, oculomotor (sphincter) roots; I, sym- * |" r ^ es \ ^on^ doses cause moderate 



pathetic (dilator) ; K, L anterior roots ; ^atetion of the pupil m consequence of paralysis 



^N.O, posterior roots; A, seat of lesion , !*&** ^ ? el1 J 8 l th ? c f onst *iug 



causing pupillary immobility; * probable Il p 1^ r wW^i f v d 8 * roctl0n t 



seat of lesion, causing myosis. F e , , clhar J [ophthalmic] ganglion (Hensen and 



' J Volckers) [and division of all the nerves except 



the optic], and in the excised eye (De Ruyter), [so that atropin is a local mydriatic. 



In moderate doses it paralyses the nervous terminations of the 3rd nerve (but not in birds 



whose iris contains striped muscle), and in larger doses it also paralyses the muscular fibres]. 



(Cocaine, or cucaine, is obtained from the leaves of Erythroxylon coca. When applied locally 



it acts as a powerful local ana?sthetic, and hence it is very useful for operations about 



the muco-cutaneous orifices. A 4 per cent, solution dropped into the eye produces complete 



insensibility of the cornea in a few minutes. It causes dilatation of the pupils, though they 



react to light and to the movements of accommodation. It also causes temporary paralysis of 



