44 GENERAL ACTIONS OF DRUGS 



Uses. — Drugs influencing tlie peripheral motor nerve- 

 endings are not of any practical therapeutical value. 



IV. — Drugs Acting on the Nerves or Special Sense. 



(A) Drugs acting on the eye, 



(1) Drugs influencing the pupil. — The mechanism controll- 

 ing the pupil consists of the centres for the contraction of 

 the pupil (in corpora quadrigemina?), the centres for the 

 dilatation of the pupil (in the medulla and aqueduct of 

 Sylvius?), the third nerve, the cervical sympathetic and the 

 circular and radiating (latter sometimes absent) muscular 

 fibres of the iris. Drugs may act either centrally or locally 

 on these structures. The pupil is dilated by drugs (1) 

 depressing the contracting (oculomotor) centre, (2) the ter- 

 minations of the third nerve or (3) the circular fibres of the 

 iris ; and contrariwise, by (4) stimulating the dilating centre, 

 (5) the terminations of the sympathetic or (6) the radial 

 fibres of the iris ; and, finally, by a combination of these 

 actions. 



Again, the pupil is contracted by drugs stimulating 

 (1) the oculomotor centre, (2) the terminations of the third 

 nerve or (3) circular fibres of the iris ; and by depressing (4) 

 the dilating centre, (5) the terminations of the sympathetic 

 or (6) the radial fibres of the iris ; and also by a combina- 

 tion of these actions. Drugs may act locally on the pupil 

 through the medium of the circulation as well as when 

 dropped into the eye. Furthermore, absorption and central 

 action may occur when drugs are dropped into the eve as 

 well as when entering the blood through the more ordinary 

 channels. 



The drugs used in the treatment of the diseases of the 

 eye are only those acting locally. Drugs influencing the 

 pupil are divided into two classes : (1) those that contract 

 the pupil (myotics) and (2) those that dilate the pupil 

 (mydriatics). 



