460 



MOTION. 



would require a great expenditure of muscular 

 force, which is avoided by the inclination of 

 the trunk. That this inclination really takes 

 place is confirmed by experience, and its 

 amount has been measured by Weber, both in 

 walking and running with various degrees of 

 velocity. In order to take these measures with 

 accuracy, a given path was passed over, some- 

 times in running and sometimes in walking, 

 with various degrees of speed, the time being 

 measured and the steps counted. By means 

 of a telescope, placed sideways to this path, 

 the difference of inclination in walking and 

 running was ascertained, but as it was extremely 

 difficult to find the actual position of the centre 

 of gravity of the trunk in such varying posi- 

 tions, a conspicuous line marked on the trunk 

 was substituted for the vertical line passing 

 from the axis of the femurs through the centre 

 of gravity, and thus its slightest deviations 

 were observed. The vertical position of the 

 trunk when at rest may be exactly estimated by 

 the angle which it makes with this marked line 

 in walking to and fro, in running to and fro, 

 and in standing in the two opposite positions. 

 The last angle, deducted from each of the two 

 first, gives the double inclination of the trunk 

 from the vertical in walking and running. The 

 results of these measurements are given in the 

 following Tables. 



TAKLE 1. 



Aleasure of the inclination of the trunk in 

 walking and running. 



Angle between the 

 two opposite posi- 

 tions of the maiked 

 line in standing. 



13 74 

 13 74 

 13o 66 

 15 46 



14" 00 



TABLE 2. 



Measure of the inclination of the trunk in 

 walking slowly. 



TABLE 3. 



Measure oj' the inclination of the trunk in 

 quick walking. 



TABLE 4. 



Measure if the inclination of the trunk 

 in running. 



We therefore conclude that when we vary 

 our steps, the velocity of the upper ends of the 

 legs, together with a corresponding inclination, 

 is communicated to the trunk. The trunk, 

 then, as has been before observed, exactly re- 

 sembles a rod balanced on the finger and 

 carried forward ; the inclination in both cases 

 depending on the laws of mechanics. 



The force of the muscles which keep the 

 trunk in a state of equilibrium is likewise eco- 

 nomized in walking and running by the regular 

 oscillations of the arms. The distance of the 

 scapulo-humeral articulation from the axis on 

 which the trunk freely moves, gives any force 

 applied at the shoulder-joint a considerable 

 mechanical effect on the trunk. In progression 

 the arms and legs move simultaneously in 

 the following order. Whilst the right leg 

 swings forwards, the trunk is turned round hori- 

 zontally on the head of the left femur, and 

 would propel the right shoulder before the left, 

 but at the same time the right arm swings back- 

 ward and the left forwards, and by generating a 

 force in an opposite direction neutralises this 

 tendency. A corresponding compensation takes 

 place when the left leg swings forward, and 

 this is effected by a good walker, without any 

 sensible lateral twisting of the trunk. The 

 length of the arms is so adjusted to that of the 

 legs, that they oscillate with the latter in small 

 curves simultaneously. Weber has computed 

 the duration of a single oscillation of the arm, 

 hanging straight down, at 0".63, and when bent 

 at right angle's at 0".53. The pace is less con- 

 strained and less fatiguing when the arms are 



