GENERAL PROPERTIES OF THE NERVES. 



595 



by means of electricity, a stimulus more closely resembling the nerve-force than any 

 other, and one which may be employed without disorganizing the nerve-tissue, and which 

 consequently admits of extended and repeated application. The action of electricity, 

 however, with the methods of preparing the nerves and muscles for experimentation, 

 will be fully considered under a separate head. 



The irritability of the motor system is entirely distinct from that of the sensory 

 nerves, and one may be destroyed, leaving the other intact. This follows almost as a 

 matter of course upon the fact of the anatomical distinction between motor and sensory 

 nerves ; but it is interesting to note the limits of the irritability after death in nerves of 

 different properties and the differences in the manner of its disappearance. The woorara- 

 poison, a very curious agent prepared by the South-American Indians, has the remarkable 

 property of paralyzing the motor nerves, leaving the nerves of sensation intact. This 

 fact has been demonstrated by Bernard and others by <very curious and ingenious experi- 

 ments. The poison, like those of animal origin, acts most vigorously after introduction 

 under the skin or absorption from wounds, and it produces no toxic effects when taken 

 into the stomach, except when introduced in large quantity in fasting animals. Under 

 the influence of this agent, an animal 

 dies with complete paralysis of the 

 motor system, presenting, among other 

 phenomena, arrest of respiration. Most 

 of the varieties of the poison affect only 

 the motor nerves and do not influence 

 the action of the heart ; and, in animals 

 brought completely under its influence, 

 artificial respiration will enable the 

 heart to continue its action, and, in 

 some instances, if this be persisted in, 

 recovery will take place. 



The fact that the woorara-poison 

 affects the motor nerves only has been 

 experimentally illustrated by Bernard, 

 taking advantage of the reflex func- 

 tions of the spinal cord to show the 

 persistence of the irritability of the 

 sensory nerves. The most striking of 

 these experiments is the following : A 

 frog is prepared by exposing the nerves 

 in the lumbar region, and then isolating 

 the posterior extremities by applying a 

 strong ligature, including the aorta and 

 all the parts except the nerves; so that, 

 practically, the only communication be- 

 tween the posterior extremities and the 

 body is by the nerves. It is evident, 

 therefore, that, if the poison be intro- 

 duced under the skin of the body, act- 

 ing, as it does, through the blood, it 

 will affect all parts except the posterior FIG. ig8. Frog prepared so as to show that icoorara de- 

 extremities: for the poison acts from strays the properties of the motor nerves. (Bernard.) 



. , A, A, lumbar nerves ; B, aorta. 



the periphery to the centres and must 



circulate in the parts to which the motor nerves are distributed. If the posterior 

 extremities be now irritated, the impression is conveyed to the spinal cord through the 

 sensory filaments of the lumbar nerves, which are intact; this gives rise to a stimulus, 



