On the Movement of Animals, Part One

They suppose that there are found in animals simple rhomboidal muscles, like ABCD, a whose tendon AC is fastened to a firm bone EAC, or is affixed to the end E; the opposite tendon BD, however, is equidistant to AC itself, and they are separated from each other; furthermore, there are two contrary powers, of which one is the weight R pulling the tendon BD downwards from B towards F, 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 action occurs through the tension of the fibers without the addition of a new body, in that no inflation, or increase of mass, or diminution is observed in them. Finally, they say that as often as in an oblique-angled prism ABDC, whose two opposite planes AC, BD retain the same measure, and the mass of the aforementioned solid is neither increased nor diminished, but only the oblique fibers AB, CD are shortened, the obliquity of the prism ABDC will necessarily be diminished, and will approach more towards the rectitude AGHC; and consequently the acute angle BAC will be increased, as is GAC; and therefore the weight R will be pulled upwards.

And this whole speculation relies upon that trite proposition of Euclid, that two prisms ABDC and AGHC, established upon the same base AC between two parallel planes, are equal to each other, and vice versa. From which it follows that the aforementioned equal prisms are not equally long, nor equally thick, such that the one that is more oblique, ABDC, is longer and narrower than the one that is less oblique, AGHC; and therefore the more the sides AB, CD are shortened, the more the thickness of the prism is increased. Let us now see whether the reasoning of the distinguished men coheres with the assumed principles and with the experiments. When the prismatic fibers AB, CD are shortened, and coincide