OPIUM AND MORPHINE 3 13 



drug upon the brain, nnd not to any direct iiifluence upon 

 the sensory nerves, which are unaffected except in the later 

 stage of poisoning. The unique and inestimable value of 

 opium depends upon its anodyne action. Depression of the 

 brain in relation to the cortical centres, is not sufficient to 

 cause paralysis in man, except in poisoning. Muscular 

 weakness is present in man and dogs, but even this evidence 

 of depression may be absent in horses and ruminants, yet 

 pain be effectually relieved. Contraction of the pupil, in 

 man and dogs, is due to stimulation of the oculomotor 

 nerve, probably through excitation of the pupillary centre. 



Dilatation, which occurs in poisoning, is due to para- 

 lysis of this centre. Initial stimulation of the vomiting 

 centre may cause emesis ; but, as depression of the centre 

 rapidly ensues, the act becomes later improbable. The 

 •motor tract of the spinal cord is first stimulated and reflex 

 excitability increased. This action, brief and slight in men 

 and dogs, is more prolonged and prominent in horses and 

 ruminants. Depression of the spinal motor cells succeeds 

 excitation, and reflex action is abolished. The excitability 

 of the motor and sensory nerves is slightly increased, but 

 otherwise the nerves are not affected except in poisoning, 

 when the sensory, and later the motor nerves are paralyzed. 

 The muscles remain uninfluenced. Depression of the 

 medulla closes the" scene, with paralysis of the respiratory 

 centre. 



Respiration. — Opium does not influence the respiratory 

 functions in small therapeutic doses, but large doses make 

 the respiratory movements slower and feebler, and death 

 occurs from the direct depressing and paralyzing action of 

 the drug upon the respiratory centres in the medulla. 



Circulation. — Small doses commonly produce little effect 

 upon the heart. Large doses first increase the force and 

 frequency of the heart's action, while toxic quantities depress 

 the organ. This result is due to a two-fold action upon the 

 circulation, including the heart muscle (or its ganglia) and 

 the inhibitory apparatus. The cardiac muscle is primarily 



