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go O B S E R V A T I O N S ON THE
C H A P T E R XXVII.
Of the Termination of the Nerves in the Mufcular Organs;
and whether Mufcles poflefs a Vis Infita different from the
Vis NeiTea.
T.
O OME eminent writers have fuppofed that the mufcular fibres are the
continuation of, or are formed by the nerves % and of late years they
have been called the moving extremities of the nerves.
But the following circumilances are inconfiilent with fuch an hypothefis.
1. Mufcular fibres, in confequence of their ofiice, have confiderable
llrength and tough nefs; whereas the nerves, even when covered with their
pia mater (which thofe very authors fuppofe them to lay afide at their terminations)
are pulpy and foft.
2. That matter which we know for certain to be medullary or nervous,
does not appear to be endowed with the power of contrafling when irritated.
3. As the nerves confift of threads laid parallel to each other, and which
do not, like the blood-veffels, divide into branches, the bulk of which
greatly exceeds that of the trunks which produce them, how are we to
conceive that a fmall nerve can form a large mafs of flefli ?
4. If
• Cole, Santorinus, Boerhaave, Inft. p. 395.
N E R V O U S S Y S T E M . 91
4. If the mufcles were formed by the extremities- of the nerves, they
ihould ihrink very remarkably on. cutting the nerves, inilead of which, I
have obferved no fcnfible alteration in the appearance of the mufcles of the
thigh and leg of a frog upwards of a year after I had cut acrofs its fpinal
marrow or fciatic nerves.
Mufcles, or mufcular fibres, feem, therefore, to be organs fui generis, not
produced by the nerves, but merely influenced by the energy they convey.
T. 11.
NOTWITHSTANDING the above conclufion, that the mufcles are machines,
the effential bafis of which is not derived from the termination of the
nerves, we ought not, I apprehend, to adopt the idea of Dr Haller, that
mufcles are not only thrown into adion by the vis nervea, but a<5t alfo by
a vis infita, differing from, and unconnected with the vis nervea.
The chief experiment which feems t6 have led Dr Haller to this opinion,
is the well known one, that the heart and other mufcles, after being detached
from the brain, continue to aft fpontaneoufly, or by ftimuh may be
roufed into aftion for a confiderable length of time; and when, fays Dr
Haller, it cannot be alledged that the nervous fluid is, by the mind, or otherwife,
impelled into the mufcle.
That, In this inftance, we cannot comprehend by what poveer the nervous
fluid or energy can be put in motion, muft, perhaps, be granted.
But, has Dr Haller given a better explanation of the manner in which his
fuppofed vis infita becomes adive ?
If it be as diflicult to point out the caufe of the aftion of the vis infita
as that of the a£lion of tlie vis nervea, the admiifion of that new power, inftead
of relieving, would add to our perplexity.
We