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 for a 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.
daring eontraction.-1. Of the changes essential to the act.—A muscle in action becomes shorter and thicker, and it is well ascertained by experiments often repeated that these changes in its relative dimensions are accurately pro portioned to one another. The whole organ neither gains nor loses in bulk.
What is true of the organ is true of the tissue—in contraction it increases in diame ter and shortens in a corresponding degree. This is all that can be said in general respect ing the visible features of this remarkable phenomenon. Late investigations, instead of explaining the manner in which contraction is effected, by sheaving its dependence on forces previously understood, have only served to point out the inadequacy of the coarse and mechanical hypotheses which physiologists have 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 as the simplest which the most refined microsco pical analysis can ever disclose.
The intimate connexion between the nerves and muscles, both in rest and action, arid 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 may be thought, therefore, a subject of disappointment that when at length a close insight into its visible characters has been obtained, and the minutest particles which the best instruments can discern have been brought under observa tion during their state of activity, the only change that can be appreciated in them is that which was long ago known by accurate experi ment to occur in the aggregate mass, viz. that
they become shorter and thicker.
All muscle, after systemic death or after removal from the body, undergoes a contraction, termed the rigor mortis, which has received much attention in all that relates to the mode of its approach, its course and duration, and the practical bearings it presents. (See DEATH.) This phenomenon may be varied by the appli cation of stimuli, and is eminently suited for the display of the minute changes occurring in muscle during its active state.
The muscle with striped fibres is peculiarly adapted for the display of these changes; for its texture not being homogeneous, but marked throughout with perfect regularity into spaces or particles so minute as to require to be very highly magnified before they can be even seen at all, the anatomist is provided with the means of detecting movements, which, without this circumstance, must have remained concealed. It is accordingly by the study of this variety of the tissue that the results just alluded to have been obtained.
When a fragment retaining its contractility is torn up into its elementary fibres, these are seen to undergo a slow movement at certain points, especially where they have suffered violence, as at their broken extremities. This movement consists of a shortening and thicken ing of t!ie material composing the fibre, as is shown by the general outline of the part, but especially by the appearances visible in its interior. The transverse stripes, both light and dark, become longer and thinner ; in other words, the discs expand in circumference, flatten and approximate to one another ; or to use another form of expression, the become shorter and thicker, both in the titles composing them and the material con necting those particles (fig. 301).
These changes are always local, or partial, and it is most evident from the characters they constantly present, that they are not limited to any determinate regions, points, or segments, but occur indifferently wherever the exciting cause may chance to be exerted. Neither discs nor fibrilla: appear to have the smallest share, as aggregations of particles bearing those par ticular forms, in producing the phenomena of contraction. A contraction is never bounded to a particular number of discs or fibrilla:, and is never accurately limited by the interval between two discs. It constantly happens that at the edge of the contracted part several discs are only partially engaged in it. A contraction generally, when commencing at the broken end of a fibre, occupies its whole width there; but when it commences at the border of the fibre it may be confined to a portion of many discs. And, further, a contraction never occu pies the whole length of a fibre or fibrilla at once. A contraction excited in an elementary fibre by the contact of a hair extends into the mass equally in all directions, as we might suppose it would do, if the mass were homo geneous.