OPIUM AND MORPHINE 339 



study the drug from the comparative standpoint in order to 

 obtain a full understanding of its effects. The brain of man, 

 being more highly developed and sensitive, in comparison 

 with other parts of the nervous system, than the brain of 

 the lower animals, it follows that this organ is more power- 

 fully influenced in man, while the spinal cord is often 

 mainly impressed in the lower animala. 



We may take the action of opium on the frog, at one 

 end of the scale, as exhibiting the most active spinal symp- 

 toms; while in man, at the other end of the scale, cerebral 

 phenomena predominate. The other animals occupy an 

 intermediate position; the action upon the horse and rumi- 

 nants is something between that exerted upon the frog and 

 man, and the influence upon dogs approaches more nearly 

 that seen in human beings, only that a relatively greater 

 dose is required to produce the same result, as the brain is 

 not so highly organized or sensitive to the action of medi- 

 cines. The brain of the horse is only one-twelfth as large, 

 in proportion to their respective body-weights, as that of 

 man, and it follows that the spinal cord of the horse is more 

 readily affected by opium, in accordance with the general 

 law that the more highly developed a part is, the more 

 easily is it influenced by therapeutic agents. 



Opium exerts first a stimulating, and then a depressing 

 action upon the brain and spinal cord, and in studying the 

 action comparatively it will be noted that the influence 

 upon the cord in the frog, horse, ruminant, and lo some 

 extent in the dog, preponderates frequently over the effect 

 of the drug upon the brain, for the reasons stated above. 



Action Upon the Frog. — In minute non-poisonous doses, 

 sleep is produced, followed by a period of reflex excitement. 

 Toxic doses of one or two grains of morphine, injected under 

 the skin, cause at first a condition where convulsions occur, 

 if the animal is artificially irritated ; later they come on 

 spontaneously. This state is followed by general paralysis, 

 respiratory failure and death. The convulsions are shown 

 to be due mainly to irritation of the spinal reflex centres, 



