The force of muscles, according to the experiments of Schwann, increases with their length, and _vice versa_. It is a curious circumstance, and worthy the attention of those interested in homologies, that the voluntary muscles of the superior and inferior extremities, and more especially of the trunk, are arranged in longitudinal, transverse, and oblique spiral lines, and in layers or strata precisely as in the ventricles of the heart and hollow muscles generally.[17] If, consequently, I eliminate the element of bone from these several regions, I reproduce a typical hollow muscle; and what is still more remarkable, if I compare the bones removed (say the bones of the anterior extremity of a quadruped or bird) with the cast obtained from the cavity of a hollow muscle (say the left ventricle of the heart of the mammal), I find that the bones and the cast are twisted upon themselves, and form elegant screws, the threads or ridges of which run in the same direction. This affords a proof that the involuntary hollow muscles supply the type or pattern on which the voluntary muscles are formed. Fig. 6 represents the bones of the wing of the bird; fig. 7 the bones of the anterior extremity of the elephant; and fig. 8 the cast or mould of the cavity of the left ventricle of the heart of the deer. [17] “On the Arrangement of the Muscular Fibres in the Ventricles of the Vertebrate Heart, with Physiological Remarks,” by the Author.--Philosophical Transactions, 1864. “On the Muscular Arrangements of the Bladder and Prostate, and the manner in which the Ureters and Urethra are closed,” by the Author.--Philosophical Transactions, 1867. “On the Muscular Tunics in the Stomach of Man and other Mammalia,” by the Author.--Proceedings Royal Society of London, 1867. [Illustration: FIG. 9.--The Superficial Muscles in the Horse, (after Bagg).]