212 MYDRIATICS 



be used as a substitute for eserine or pilocarpine for these conditions. 

 Contraindications. Arecoline is contraindicated in old debili- 

 tated animals, those with a weak heart, in nervous (spasmodic) colic, 

 pharyngitis, diseases of the lespiratory tract and in pregnancy. It 

 should be administered to cattle with caution. 



MYDRIATICS 



Mydriatics are agents which dilate the pupil. The size of the 

 pupil is regulated by two sets of muscles in the iris, circular which 

 contract the pupil and radiating, which dilate it. The circular 

 muscle is supplied with the third cranial nerve and the radiating by a 

 branch of the sympathetic. Mydriasis may be brought about in any 

 of the following ways : 



1. Stimulation of the sympathetic center. 



2. Stimulation of the peripheral fibers of the sympathetic in the 

 radiating muscle. 



3. Stimulation of the radiating muscle itself. 



4. Paralysis of the oculomotor center. 



5. Paralysis of the endings of the oculomotor nerve in the circular 

 muscle. 



6. Paralysis of the circular muscle itself. 



Mydriatics interfere with the power of accommodation by a paresis 

 of the ciliary muscle. Atropine, hyoscyamine and hyoscine -are most 

 powerful in this respect, while cocaine and euphthalmine are least so. 

 Intra-ocular tension is increased, probably on account of the narrow- 

 ing of the angle between the iris and cornea, thus hindering the es- 

 cape of the humor through the canal of Schlemm or by blocking the 

 efferent lymph vessels. Cocaine and euphthalmine are least active in 

 this respect. 



Therapeutics. 



1. Facilitate examination of the interior of the eye. 



2. Paralyze accommodation in examination for refractive errors 

 in human medicine. 



3. Prevent and break down adhesions of the iris. 



4. Eest the iris. 



5. Enlarge field of vision, where there is nticlear cataract with 

 clear periphery of the lens. 



6. Allay irritation in inflammation of the cornea. 

 The most important mydriatics are: 



Atropine Hyoscyamine 



Homatropine Cocaine 



Methylatropin (Eumydrin) Euphthalmine 

 riyoscyine or Scapolomine 



