522 
motor nerves, which are thus made to excite 
contraction by their immediate and topical 
action on the muscles. Of these the chief are 
volition, emotion, and impreasions carried by 
the afferent nerves to the nervous centres and 
involuntarily reflected thence; but various 
diseases and injuries of the motor nerves, either 
at their origin or in their course, and pressure, 
heat, chemical substances, electricity, gal- 
vanism, &c. applied to their texture, are to be 
ranged under the same head. 
The nature of the change thus induced in 
the motor nerves is entirely unknown. There 
seems, however, no ground for believing that 
it differs with the particular stimulus which 
induces it, and certainly a clear distinction 
ought ever to be drawn between it and its 
exciting cause. The nerves, when this change 
is induced in them, occasion the muscles to 
which they are distributed to contract. The 
stimulus they give is an immediate one, and is 
termed the vis nervosa or the nervous stimulus 
of muscle. It acts topically on the muscular 
fibre. The other immediate stimuli are physi- 
cal and chemical; they are rarely exerted in 
the living body, except in the case of the 
hollow muscles. It has already been stated 
that trustworthy experiments have lately shown 
these to be under the influence of motor nerves 
derived from the spinal marrow, but it seems 
probable that some at least are norma!ly excited 
to contract by direct stimulation, to one form 
of which, that of stretching or distension, they 
are peculiarly liable from their arrangement as 
investments to cavities. All muscles, however, 
may be made to contract by physical and 
chemical stimuli applied to their fibres. 
The effects of water and mechanical pressure 
as immediate stimuli have been already alluded 
to. Chemical substances may be seen to act 
similarly, if they be not so powerful as to 
destroy the texture of the part; and it is pro- 
bable that electrical forces have a like agency. 
An interesting phenomenon has been pointed 
out by Dr. Stokes,* which seems to show very 
clearly that contractions of voluntary muscles 
may be excited by an immediate stimulus in 
the living body. In various cases of phthisis, 
and in others, particularly those attended with 
emaciation, a sharp tap with the fingers on any 
muscular part is instantly followed by a con- 
traction, evidenced by the rise of a defined 
firm swelling at the point struck, enduring 
several seconds before it gradually subsides. 
This is often so prominent as to throw a shadow 
along the skin, and for the moment it might 
almost be mistaken fora solid tumour. That 
it is limited to the point struck is full proof of 
its being a direct effect of the irritation, and 
not produced through the intervention of 
nerves; for a contraction excited in the latter 
mode would be diffused over the part to which 
the nervous twigs irritated were supplied, and 
would therefore frequently extend to some dis- 
tance. 
c. On the visible changes occurring in muscle 
* On Diseases of the Chest, p.397. See also 
Dr. Guy, in the new edition of Hooper’s Phy- 
sician’s Vade Mecum, p. 92. 
MUSCULAR MOTION. 
during contraction.—1. Of the changes essential — 
to ceeetasal muscle in action becomes short _ 
and thicker, and it is well ascertained by 
experiments often repeated that these changes 
in its relative dimensions are accurately pros — 
portioned to one another. The whole organ — 
neither gains nor loses in bulk. oe 
What is true of the organ is true of the 
tissue—in contraction it increases in diar 1 
ter and shortens in a corresponding degree. 
This is all that can be said in general respect- 
ing the visible features of this rem cable 
Ehepeensice. Late investigations, instead of — 
explaining the manner in which contraction 
is effected, by shewing its dependence on 
forces previously understood, have only served — 
to point out the inadequacy of the coarse and 
mechanical hypotheses whi ysiologi ». 
been so prone to confide in, and to make it 
more than probable that they must ever be — 
content to repose upon the fact above stated 
the simplest which the most refined microseo- 
pical analysis can ever disclose. mg 
The intimate connexion between the nerves 
and muscles, both in rest and action, and the 
exquisite organization displayed in the structure 
of those muscles which are most quick and 
energetic in their movements, have powerfully 
contributed to excite the hope that a clue to the 
discovery of the physical mechanism of con- 
traction would one day be found. It mayb 
thought, therefore, a subject of disappointment 
that when at length a close insight into it: 
visible characters has been obtained, and the 
minutest particles which the best instrument 
can discern have been brought under observa- 
tion during their state of activity, the only 
change that can be appreciated in them is th 
which was long ago known by accurate exper : 
ment to occur in the aggregate mass, viz. th | 
they become shorter and thicker. 4 . 
All muscle, after systemic death or ¢ 
removal from the body, undergoes a con 
termed the rigor mortis, which has 
much attention in all that relates to the 
¥ 
zo 
of its approach, its course and duration, at 
the practical bearings it presents. (See Deat 
This phenomenon may be varied by the apy 
cation of stimuli, and is eminently suited 
the display of the minute changes occurt 
in muscle during its active state. _ 
The muscle with stri fibres is peeulis 
adapted for the display of these changes; 
its texture not being homogeneous, but mar 
throughout with perfect regularity into spé 
or particles so minute as to require to bet 
highly magnified before they can be even see 
all, the anatomist is provided with the m 
of detecting movements, which, without 
circumstance, must have remained conces 
It is accordingly by the study of this ¥ 
of the tissue that the results just alluds 
have been obtained. ee 
When a fragment retaining its contracti 
is torn up into its elementary fibres, these 
seen to undergo a slow movement at cert 
points, especially where they have suff 
violence, as at their broken extremities. 1 
movement consists of a shortening and thick 
