106 ACTION OF MEDICINES AND OF POISONS. 
“ The action of medicinal substances must not be viewed 
in a different light; it resembles, in every respect, that of 
poisons. Nerves, muscles, blood-globules, and other tissues, 
enjoy peculiar vital properties, the intensity of which is 
modified through medical interference, sometimes for the 
purpose of increasing, sometimes for that of lowering, their 
activity; and the action of foreign bodies, whether medicinal 
or poisonous, can be rendered general only through the 
nervous or vascular system. But, in this respect, a wide 
distinction arises between sensitive and motor fibres, which 
experiments on living animals have placed in a strong light. 
Let woorara be injected into an animaPs veins, or simply 
introduced under the skin after the vessels of one of its 
extremities have been tied, the motor nerves are paralysed 
throughout the system, and it becomes impossible to produce 
muscular contraction by acting upon them ; but the nerves of 
that single limb, the vessels of which have been tied, alone 
retain their physiological power, and provoke contraction 
when galvanized or otherwise excited. It is therefore clear 
that motor nerves give merely birth to local phenomena, and 
are incapable of acting reciprocally upon each other, through 
the central axis, in which they all unite. Far different is the 
mode of action of the sensitive fibres: let strychnia, or any 
other poison which holds them under its influence, be intro¬ 
duced into the circulating fluid, after tying, as before, the 
principal vessels of one limb, and it will be found that the 
operation has been performed in vain, convulsions being pro¬ 
duced in that very limb, through mere nervous agency, as 
well as on other points. Supposing all the posterior roots 
which arise from the spinal cord to have been divided, no 
convulsions, of course, would be produced ; but let a single 
root be left untouched, and through this single channel the 
necessary impulsion will be given ; the blood which circulates 
throughout the system, and conveys the toxic substance into 
every part, brings it in contact with the extremities of that 
sensitive root, which alone remains uninjured, and still com¬ 
municates with the spinal cord; and general effects are at 
once produced. How different, therefore, do we find the part 
played by sensitive and motor nerves to be ! 
“ It would therefore appear that the various substances 
which the physician employs exert their influence, not upon 
organs, but upon tissues; not upon the apparatus itself, 
taken as a whole, but upon some of the histological elements 
of which it is composed. It would be an entirely mistaken 
notion to suppose that digitalis paralyses the heart; not upon 
the heart itself, but upon the muscular fibres which enter 
