§i2 On the Nervous Systems of 
Dr. Monro,* seem to prove fully that the coti- 
traction of a muscle, which has been attributed to 
this supposed vis insita, may be equally well ac- 
counted for from the nervous power alone, and 
that the former would be a superfluous power 
in the animal economy, the supposed vis insita 
being excited, or destroyed by the same meansas_ 
the nervous power. When the brain or spinal 
marrow is irritated, the muscles or muscular . 
fibres contract, tremble, or are convulsed; when 
the point of a needle or other sharp body, is 
pushed into a nerve, distributed to a particular 
muscle, a contraction of that muscle ensues; in 
which cases the muscular fibres act confessedly 
by virtue of their nervous power. When a 
needle is pushed into the fibres of the muscle 
itself, a contraction also takes place, and in this 
case it has been supposed by the celebrated 
Haller and others, that the fibres contract by 
virtue of their vis insita: But the only differs 
ence appears to consist in this; that in the for- 
mer experiments the stimulus acts upon the 
brain, spinal marrow or the trunk, or an evident 
branch, of a nerve,whilst in the latter it acts upon 
one, or more of those nervous filaments, or por- 
tions of nervous matter, which, though not always 
demonstrable to the sight, we have every reason 
* Observations on the Nervous System, p, 91—94- 
