860 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



surface placed upon the head is found to write a very complicated curve. 

 There is a normal sway for every individual, and this may become markedly 

 exaggerated under pathological conditions. The maintenance of equilibrium 

 requires that afferent impulses shall continually pass to the co-ordinating cen- 

 tres which control the muscles involved in this act, and if any of these normal 

 impulses fail the sway of the body is increased ; for example, it is more diffi- 

 cult to stand steadily when the eyes are closed than when they are open ; the 

 absence of the normal sensory impulses from the skin of the feet, the muscles, 

 joints, etc., also makes standing more difficult and tends to increase the sway. 

 The effect of the normal sway of the body is to shift the pressure and strain 

 from point to point and to relieve the different muscles from continuous action. 



Locomotion. 1 The movements of animals were first studied by careful 

 observation, accompanied by more or less accurate direct measurements, and 

 by these simple methods the Weber brothers 2 arrived at quite accurate con- 

 clusions as to the nature of the processes, walking, running, jumping, etc. 

 These results were greatly extended by Marey, 3 who employed elaborate 

 recording methods, and exact pictures of all stages of these processes were 

 later obtained through the remarkable revelations of instantaneous photog- 

 raphy. 4 



Walking. During the act of walking, at the same time that the body is 

 propelled forward it is continually supported by the feet, one or the other of 

 which is always touching the ground. Preparatory to beginning the move- 

 ment the weight of the body is thrown upon one leg, while the other leg is 

 placed somewhat behind it, the knee and ankle being slightly flexed. At the 

 start the body is given a slight forward inclination, then the back leg is ex- 

 tended and impels the body forward. As the centre of gravity progresses so 

 as to be no longer over the supporting leg, it would fall were it not that the 

 back leg is at the same instant swung forward to sustain it. As the body 

 moves forward and its weight is received by the leg which has just been 

 advanced, the leg which has been its support is freed from the weight 

 and becomes inclined behind it. This leg and foot are next extended, the 

 body thereby receiving another forward impulse, and then the hip-, knee-, 

 and ankle-joints flexing slightly, the leg swings forward past the supporting 

 leg and again becomes the support of the body. The forward movement of 

 the body is due in part to a slight inclination which tends to cause it to fall 

 forward, and in part to a push given it by each leg in turn as it leaves the 

 ground. ^, 



The amounrof work performed by the legs in ordinary walking is com- 

 paratively slight, since the swing of the leg is, like that of a pendulum, largely 



1 Beaunis : Physiologic humaine, 1888, vol. ii. p. 269, gives many references to the litera- 

 ture of this subject. 



2 W. and E. Weber : Mechanik der menschlichen Gehewerkzeuge, 1836. 

 8 La Methode graphique, 1885. 



4 Marey : Methode graphique (supplement), 1885 ; Muybridge : The Horse in Moti&n, as Shown 

 by Instantaneous Photography, 1882. 



