60 HUMAN BIOLOGY 



or myomeres, arranged in a closely packed series on either 

 side of the body, quite in the manner of the fleshy muscle 

 flakes in modern fish. This stage is fully realized in Amphi- 

 oxus, the very lowest of the fish-Hke chordates still existing. 

 In normal fish-like vertebrates that move through the water, 

 the body is "stream-lined," with a rapidly widening "enter- 

 ing angle" and a long sloping "run." In forward locomotion 

 the muscle segments on one side, immediately behind the 

 head, begin to contract first and the contraction is then 

 passed backward toward the tail. Meanwhile the muscle 

 segments on the opposite side, immediately after the initial 

 contraction, begin their contraction, so that the head is 

 alternately bent slightly from one side to the other as the 

 waves of contraction run along the body, with increasing 

 ampHtude, to the tail. Presumably the continuous notochord 

 acts as an axial rod or spring. At first the wriggling body 

 does not need any accessory steering or propelling structures, 

 but in later stages folds of skin, originally not in themselves 

 movable, grow out and serve as keels and rudders. From 

 this relatively simple beginning, as we shall presently see, 

 the evolution of the locomotor apparatus, at least in its main 

 outlines, from fish to man is fairly clear. 



But before proceeding to the higher stages let us return 

 to the basic patent, the contractile muscle pouch. Why was 

 it contractile? Each muscle segment of an adult fish (Fig. 3) 

 consists of a zigzag of striped muscle fibers fastened at 

 either end into the connective-tissue septa that divide the 

 muscle segments. It is the striped muscle fiber, then, that 

 is the smaller unit of contractility. But what, in turn, makes 

 it contract? Under high magnification a striped muscle 

 fiber is seen to be composed of two different kinds of material, 

 represented by the dark and the light-colored cross-stripes. 

 Physiologists (Martin) tell us that when a muscle swells 

 out and shortens, the force at work is analogous to the 

 force of surface tension and that the frothy nature of pro- 

 toplasm supplies a relatively enormous surface for the 

 operation of the forces of surface tension exerted between 

 the materials of the dark and the light bands of the muscle. 

 But how does the nervous discharge from the central nervous 

 system release the forces of surface tension which had up to 



