508 Dynamic Theory. 



fatty medullary sheath and is more exposed to the contact of the blood 

 containing the poison. The poisoning at this point, therefore, has the 

 same effect as cutting the nerve in two. It disconnects the circuit and 

 breaks the current. The poisons nicotine and conine ( tobacco and hem- 

 jock ) have the' same effect as curare. The effect of strychnine is to in- 

 tensify the action of the nerves in converting and transmitting the stim- 

 ulating action. They become so sensitive to external irritation that the 

 slightest stimulus, even a breath on the skin, is sufficient to stimulate 

 into tetanus all the muscles of the body. (Rosenthal.) 



The nerves heretofore chiefly considered, are the motor nerves for 

 moving the muscles. The office of the vaso motor nerves, which act on 

 the smooth muscles, is similar to that of the motor nerves. They ex- 

 cite contractions in the muscle fibres whereby the vessels, of which they 

 are a part, become reduced in diameter. The nerves which enter the 

 glands produce a different effect ; and so do the afferent or sensory 

 nerves which connect the sense organs with the central ganglions of the 

 brain and spinal cord. Yet there is no anatomical difference peculiar 

 to any of them, to be discovered between the various nerves, whatever 

 their office. The effect of irritants on detached nerves, is the same on 

 all, but the manner in which the organs in which the nerves terminate 

 are affected, differs according to the nature of such terminal organs. 



Nerve fibres, like muscle fibres, are positive in the middle and nega- 

 tive on the ends. When a nerve is agitated, no perceptible molecular 

 change takes place, and the only evidence of such agitation, accessible 

 to the observer, is that which he gets by testing the electric condition 

 of the nerve. By this means it has been ascertained that when a nerve 

 is irritated at any point between its ends, the irritation travels in both 

 directions along such nerve. If a motor nerve be agitated, its proper 

 muscle will be contracted, and the stimulation will also flow backward 

 toward the brain, without, however, exciting sensation there. If a sen- 

 sory nerve, for example one of touch, be stimulated, the action on the 

 brain will cause a sensation of touching something, but the backward 

 flow toward the organ of touch will produce no effect in it. 



Wier Mitchell says, " If we f aradise 1 the track of the nerves in or 

 above the stump " ( after a hand has been amputated ), "we may cause 

 the lost fingers and thumb to seem to be flexed or extended. " . . "In 

 a case of amputation of the shoulder-joint, in which all consciousness 

 of the limb had long since vanished, I suddenly faradised the brachial 

 plexus, when the patient said at once, < My hand is there again, it is 

 bent all up and hurts me.'" "Persons who have had an arm ampu- 

 tated, are frequently able to will a movement of the hand, and appar- 

 ently to execute it to a greater or less extent. A small number have en- 

 1 Irritate by galvanic induction current. 



