510 PHYSIOLOGICAL PHYSICS. [Chs P . XLIIT. 



acting in the line b"b' ; therefore, X=P X &B. The 

 distance of the weight becomes CB. The moment of 

 the forces is consequently, affected by the positions. 



We thus see that when the arm is straightened 

 the moment of the biceps is at its smallest, and that 

 its greatest moment is when the fore-arm is at right 

 angles to the upper arm, when the muscle acts more 

 perpendicularly upon the radius. It is to be noted, 

 however, that the loss of power due to the great 

 obliquity of the muscle with the fore-arm at an open 

 angle is counterbalanced to some extent by the fact 

 that the biceps is stretched more fully, and has the 

 whole of its contraction to perform. 



Standing-, walking, etc. - The dynamical 

 principles that have been briefly referred to in pre- 

 vious chapters are capable of explaining the mechanics 

 of standing, sitting, etc. ; as also of walking, and other 

 movements of locomotion. 



In standing erect the first condition of equilibrium 

 is fulfilled, viz. the vertical from the centre of gravity 

 falls within the base of support. According to E. 

 Weber, the position of the centre of gravity of the 

 body as a whole is in the vertebral canal, near the 

 level of the upper border of the second lumbar ver- 

 tebra. In the erect posture, therefore, the vertical 

 through it falls between the two feet. But when the 

 feet are close together, the base of support is com- 

 paratively small, and a slight movement to one side 

 or other will throw the vertical outside of the 

 line, when the tendency will be to fall. The body is 

 not, accordingly, in stable equilibrium. The erect 

 posture, especially the military posture, is not one 

 which is maintained without a considerable amount 

 of muscular effort, the tendency being for the body to 

 fall forward, a tendency which is met by the resis- 

 tance of the muscles of the calf. For this reason, 

 maintaining the erect position is more tiring than 



