DESCRIPTIONE ET USU.

PROPOSITIO IV.

The action of a muscle is contraction.

In a muscle we see that only the fleshy filaments AB, CD, EF & C of the same figures 1, 2, 3, 4 of TAB. I are shortened when the muscle acts. The extreme tendons BH, to which the fleshy fibers are attached, are not contracted, but retain the same length they had before. This is evident to the sense in the anatomy of the living.

Hence it follows that only the fleshy fibers AB, CD, EF, GN & C exert force by suspending heavy weights through the energy by which they are contracted. The tendons BH, however, suffer force insofar as they are pulled by simple local motion by the contracted fleshy threads, and they serve them as handles to which the fibers are attached.

PROPOSITIO V.

A critique of the recently published structure of the muscle and its mode of operation.

In these last years, a new idea has appeared regarding the true form of the muscle and its mechanical mode of operation, regarding which we shall expose our opinion for the love of truth. TAB. I, Fig. 5, 6, 7 & 8.

They suppose that there exist in animals simple rhomboid muscles, as ABCD in Fig. 5, TAB. I, whose tendon AC is attached to a firm bone EAC, or is affixed to the end E; while the opposite tendon BD is equidistant to AC itself, and they are separated from one another. Afterwards, there are two contrary powers, one of which is the weight R pulling the tendon BD downwards from B towards F, and the other is the contractive force of the fibers, which acts by pulling the weight R obliquely upwards from B towards A, and from D towards C. They also suppose that such an action occurs through the tension of the fibers without the addition of a new body, for the reason that no inflation, increase of mass, or decrease is observed in them. Finally, they say that as often as the obliquity of an oblique-angled prism ABDC, whose two opposite planes AC and BD retain the same measure, and the mass of the aforementioned solid is neither increased nor diminished, but only the oblique fibers AB and CD are shortened, the obliquity of the prism ABDC must necessarily be diminished and will approach more towards the straightness AGHC; consequently, the acute angle BAC will be increased, as is GAC; and therefore the weight R will be pulled upwards.

And this entire speculation rests on that well-worn proposition of EUCLID, that two prisms ABDC & AGHC situated upon the same base AC between two parallel planes are equal to each other, and conversely. From which it follows—