1 90 MATERIA MEDIC A AND THERAPEUTICS. 



The grey matter of the spinal cord is at first, and briefly, 

 stimulated by a moderate dose of opium, reflex excitability 

 being increased, as shown by restlessness in man and convul- 

 sions in animals. At the stage of cerebral depression, languor 

 and muscular weakness, of spinal origin, set in, and the sub- 

 ject lies down; but there is not even then complete loss of 

 muscular power and irritability ; and even in dangerous poison- 

 ing by opium the patient can be marched about, if supported on 

 either side. 



Following close upon the convolutions and cord, the great 

 vital centres in the medulla are markedly affected by opium. 

 Vomiting is not uncommon as one of the first effects. The 

 respiratory centre, at first unaffected, is then depressed, the 

 respiratory movements becoming quiet, superficial, and ir- 

 regular ; and death by opium poisoning is due to paralysis of 

 the respiratory centre and arrest of breathing, that is, to 

 asphyxia. The cardiac centre is more resistant to morphia, 

 and is first excited so as to increase inhibition (after an 

 evanescent acceleration) ; but it is soon depressed, and the 

 pulse rises in frequency. The vascular centre is depressed by 

 opium, but never to a dangerous extent ; and even in complete 

 narcosis, when respiration is failing, the blood pressure (pulse) 

 responds to afferent stimuli. The full action of opium on the 

 respiration, heart, and vessels will be immediately described. 



We shall presently find that the therapeutical value of 

 the action of opium on the central nervous system lies in the 

 fact that it depresses the perceptive and sensory centres so 

 much earlier and more profoundly than the vital centres in 

 the medulla. Its effect on the pupil, heart, vessels, respira- 

 tion, and cord are either of little positive value in treatment, 

 or are altogether unfortunate. 



The functions of the sensory nerve terminations are lowered 

 or arrested by opium, common sensibility being especially re- 

 duced, so that pain cannot be originated ; but this peripheral 

 anaesthetic or anodyne effect of morphia given by the mouth is 

 decidedly secondary, both in time and in degree, to its allied 

 action on the sentient centres, and to its local effect when 

 administered by hypodermic or interstitial injection, already 

 described. 



The sensory nerve-trunks are diminished in conductivity by 

 local injection of morphia, as well as by its internal administra- 

 tion, thus offering a second interruption to the flow of painful 

 impressions inwards. 



The motor nerves are first briefly excited, and then para- 

 lysed from the centre outwards. Muscular irritability is never 

 completely lost. 



