THE HUMAN STRUCTURE 107 



Three rotations round three rectangular axes are therefore 

 possible, and this is seen in the hip and shoulder joints 



These three axes are instantaneous axes, but taking a bone 

 in repose, the thigh, for instance, one can see that the axis 

 of a limb allows of a vertical torsion or rotation ; i.e., there is a 

 frontal axis round which the limb bends, and a sagittal axis for 

 the lateral movements. The amplitude of the lateral movement 

 is about 90 ; that of the shoulder is 150. 



(iv) When two or more joints move together, the degree of 

 freedom increases. This is what occurs in the movements of the 

 hand, mobile from the wrist, which has numerous jointed bones 

 (fig. 116) and in those of the vertebral column, etc. 



One cannot here enter into the details of these " combinations " 

 and " compositions " of simple joints ; it will only be pointed 

 out that the greater their freedom of movement the more are the 

 ligaments, tendons and muscles, relieved of an important part of 

 their effort, and this serves to economise force in the human 

 machine. 



76. The Human Body, (i) Shape. The various parts which 

 we have examined constitute the organs of locomotion ; but 

 the functions which maintain movement are those of respiration, 

 circulation and innervation of which the seat (except the cerebral 

 centres) is the trunk ; all the digestive apparatus being in the 

 lower part of this trunk. The head contains the directing organs, 

 the senses and the will. And from this complex whole results 

 an admirable harmony. The shape is the least complex ; it 

 approaches that of the " prism." The human body feels the 

 effect of its own weight, and it has been shown that in prismatic 

 or cylindrical shapes the section of the least resistance is about 

 at " the middle of the height/' It is there that the weight of the 

 body is felt the most, but it is there also that the body is 

 enlarged, the " pelvis " receiving at that level a veritable re- 

 inforcement, and the strength, " according to ancient wisdom, 

 lying in the loins." But an exaggerated development of the 

 iliac bones would inconvenience walking, in producing rotations 

 of the body (see 270) . People who lead normal lives, free from 

 extraordinary exertion of the lower limbs, have a relatively 

 narrow pelvis, in contrast to heavy athletes who have wide 

 and very muscular hips. 



Dynamic actions tend to develop or refine the body ; carters, 

 dock labourers, etc., being often massive ; dancers, runners, 

 fencers often being slender. The carrying of loads modifies the 

 shape of the limbs ; for example, walking under a load results 

 in flattening the " foot " and elongating it permanently. It is 

 the same with the hand in the handling of heavy tools (hammer, 

 spade, pick, etc.) ; the axis of the body, the vertebral column 



