THE MECHANISM OF MOVEMENT 527. 



The muscular work done by a man is calculated by multiplying the 

 weight lifted by the height of the lift. Thus 2 kilogrammes lifted 

 through 2 metres gives 4 kilogramme-metres of work. When a 

 man runs upstairs very fast he may, in lifting his body, do seventy 

 times more work in a minute than a navvy does in the same time 

 who is steadily shovelling up earth. The man, however, is spent at 

 the end of such an effort ; the navvy can continue to shovel leisurely 

 for hours. 



Excess either of rate of work or of load will lessen efficiency and 

 diminish output. Scientific management determines the suitable rate 

 and load for each kind of labour. 



The Erect Posture. With the assumption of the erect posture one 

 of the chief functions of the system of levers of the human body 

 became that of .maintaining the centre of gravity of the body within 

 the body. Since the centre of gravity of a body always tends to take 

 up the lowest possible position, it must lie over the base of support, 

 otherwise the body will topple over. 



A dead man cannot, without support, be made to stand in the 

 erect posture. If a man standing erect faints, the head tends to fall 

 forward on the chest, the trunk forwards at the hip-joints, and the 

 whole body forwards over the ankle-joints. Although the body is 

 balanced by muscular action, the weight of the body is borne by the 

 bones and ligaments, and thus fatigue is avoided. In the stork the 

 bones of the leg can be so locked together to balance the body that 

 the bird can sleep restfully standing on one leg. In man the main- 

 tenance of the erect posture is more of an effort, so that for this, as 

 well as other reasons, he seeks rest in the recumbent posture. 



The body is maintained erect by the following means: 



The head is balanced by the muscles so as to rest on the top of 

 the vertebral column. As the centre of gravity lies in front of the 

 joint, the head tends to fall forwards in a sleepy man; the neck muscles 

 must act to keep it from doing so. The vertebral column forms an 

 elastic rod supporting the head and trunk ; below it is fixed immovably 

 to the broad pelvic basin, into which presses the weight of the abdo- 

 minal organs. The centre of gravity of the body is situated near 

 the front of the last lumbar vertebra. If a plummet-line could be 

 dropped from the centre of gravity, the line would pass a little behind 

 the line which joins the two hip- joints. The trunk thus tends to 

 fall backwards at the hip-joints; this is prevented by the strong 

 ligament which passes from the pelvis to the femur across the front of 

 each joint. Thus the joint is locked and the muscles passing from 

 the trunk to the thighs have simply to balance the body upon the 

 heads of the thigh-bones. To do this but little effort is required. At 

 the knee the plummet-line dropped from the centre of gravity would 

 pass through a line joining the posterior parts of both joints. The 

 weight of the upper part of the body thus presses upon the flat articular 

 surfaces of the tibiae. The great extensor muscles in front of the 

 thigh prevent the knees from bending, and the body from falling 

 backwards whenever balance is disturbed. Owing to the check liga- 



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