270 



ANIMAL MECHANICS. 



taken. Moreover, it is a well known and undoubted fact that people 

 with short legs walk in quicker time than those with long ones, when 

 they assume a natural pace, as free as possible from muscular exertion. 



The investigations of Marey just quoted have, however, shown that 

 there is no necessary relationship between the length and the duration 

 of the stride. 



Mechanical forces operative during progression. In walking 

 and running there are certain forces which have to be so equilibrated 

 that the pace is neither increased nor diminished, and that the body 

 retains its elevated position from the ground. There is, in the first 

 place, the weight of the whole mass of the body tending to seek its 

 lowest level. Secondly, there is the resistance of the air, which will 

 vary with the velocity of progression, and the extent of surface pre- 

 sented by the body, etc. ; and, thirdly, the reaction of the ground upon 

 which progression is performed. As against these forces we have the 

 muscular force of the body which tends to straighten the legs, raise the 

 a centre of gravity, and give it the necessary 

 forward velocity. 



During forward progression, it is certain that 

 the centre of gravity is constantly and rhyth- 

 mically changing its position, as the body itself 

 is mobile, and rhythmically changes its shape. 

 The centre, which always lies approximately in 

 the median sagittal plane of the body, is moved 

 in space on a horizontal plane, not only forwards 

 but from side to side. This side-to-side move- 

 ment is due to the shifting of the body in the 

 direction of the leg on which it for the moment 

 rests. Marey, by photographing from the front 

 a bright spot over the pubes, showed this side-to- 



FIG. 160. The upward side movement of the body; and the same can 

 thrust of the leg is repre- experimentally be shown by walking towards two 



From* the' ceTtr'e 'of vertical threads P laced in a line with ne e 7 e > ne 



thread under these conditions appears alternately 

 O n each side of the other. These movements are 



of 



gravity, the linecfc repre- 

 sents the force of gravity. 



By completing the paral- slight) and are usually disregarded in discussing 



the mechanism of locomotion. In relation to 

 other parts of the body, it is certain that the 



lelogram cadb, we obtain 

 the line cd which repre- 

 sents the displacement of 



ethat will actually take ' '"^ ^ "^^ty shifts' i ts " position during pro- 

 gression, coming forwards in the mesial plane 



when the body is bent, and probably moves rhythmically to some slight 

 extent as the limbs swing. These movements also we must discard as 

 being relatively unimportant, and we must view the centre of gravity as 

 fixed in the body, and in the median plane just above and somewhat in front 

 of the middle of the line joining the centre of rotation of the hip-joints. 

 The chief movements in space of this centre which we may fix by the 

 movements in space of the hip-joint are a movement forwards and a 

 movement up and down. 1 Marey's chronophotographic figure (Fig. 158) 

 shows that the hip and shoulder, and therefore the centre of gravity, 

 as well as moving forwards, are displaced upwards and downwards 

 several centimetres during each stride. The moment at which the leg 



1 During walking the mean position of the centre is nearer the ground than it is in 

 the erect position. It is still lower during the act of running. 



