166 SPECIAL PHYSIOLOGY. 



progression upon water in air; progression in water; and lastly, pro- 

 gression in the air alone ; but locomotion on solids, locomotion on or 

 in water, and locomotion in the air, constitute the chief forms of pro- 

 gression. 



In ordinary locomotion upon solids, the weight of a man or animal 

 is supported on those bodies ; and in moving upon them, the centre 

 of gravity of the living animal is invariably raised from the base of 

 support, as it is moved onwards, and then descends again, so as to de- 

 scribe a curved line. The muscular force necessary to move the ani- 

 mal, operating through the lever-like bones, impinges on the solid 

 basis of support ; and, as this is assumed, in the simplest case, to be 

 practically immovable, the force exerted acts upon the animal itself, 

 and so lifts it, and moves it onwards in space. Supposing the base of 

 support to be more or less yielding, as in the case of soft ground, or 

 of the flexible twig of a tree, then a certain amount of the force ex- 

 erted by the animal, is lost in disturbing the basis of support, a part 

 only being left to accomplish the movement of the animal. 



In progression upon a solid body, performed under water, the same 

 principles are involved ; but the muscular effort required to lift the 

 centre of gravity of the animal, is much less than in locomotion upon 

 a solid support in the air ; because the weight of the animal is par- 

 tially sustained by the hydrostatic pressure of the water ; and only 

 that part of its weight, which is in excess of the weight of an equal 

 bulk of water, has to be lifted up from the base of support. At the 

 same time, the surrounding medium being heavier than air, a greater 

 resistance is offered to any onward movement ; a condition which 

 more than neutralizes the advantage just named, and which renders it 

 more difficult to move rapidly at the bottom of a river, than to run in 

 the air. 



In progression upon a fluid, as in the case of a swimming bird, the 

 conditions of support are those of partially submerged or floating 

 bodies, and are quite peculiar. The weight of the animal is entirely 

 supported by the displacement of a quantity of water of equal weight ; 

 and no effort on the part of the animal is necessary to sustain it above 

 its base of support. All its power is, therefore, free to be exerted in 

 progressive movement, which, however, is performed at a disadvan- 

 tage ; first, because of the resistance offered by the displaced water to 

 the submerged part of the body ; and, secondly, and this is more im- 

 portant, on account of the imperfect stability of the medium against 

 which the muscular force acts, for a very large part of that force 

 operates merely in putting that medium into partial motion, whilst 

 only a small portion acts, or rather reacts, from the imperfectly re- 

 sisting medium, to set the body of the animal in motion through it. 

 Hence, the movement of an animal upon water, can never be so rapid 

 as the movements of certain animals over the land. 



In progression in water, the weight of the animal, as in the case of 

 fish, is mainly supported by the equal hydrostatic pressure of that 

 medium ; and only that smaller portion of weight, which is in excess of 

 the weight of its own bulk of water, operates so as to make it descend 



