The simplest type of vertebrate animal, the fish, has a mode of locomotion which involves alternating transverse strains. It is not, indeed, subjected to alternating transverse strains by some outer agency, as in the case we have been investigating: it subjects itself to them. But though the strains are here internally produced instead of externally produced, the case is not therefore removed into a wholly different category. For supposing Fig. 284 to represent the outline of a fish when bent on one side (the dotted lines representing its outline when the bend is reversed), it is clear that part of the substance forming the convex half must be in a state of tension. This state of tension implies the existence in the other half of some counter-balancing compression. And between the two there must be a neutral axis. The way in which this conclusion is reconcilable with the fact that there is tension somewhere in the concave side of a fish, since the curve is caused by muscular contractions on the concave side, will be made clear by the rude illustration which a bow supplies. A bow may be bent by a thrust against its middle (the two ends being held back), or it may be bent by contracting a string that unites its ends; but the distributions of mechanical forces within the wood of the bow, though not quite alike in the two cases, will be very similar. Now while the muscular action on the concave side of a fish differs from that represented by the tightened string of a bow, the difference is not such as to destroy the applicability of the illustration: the parallel holds so far as this, that within that portion of the fish’s body which is passively bent by the contracting muscles, there must be, as in a strung bow, a part in compression, a part in tension, and an intermediate part which is neutral.