THE SKELETON AND MUSCLES 



IO 



10 

 10 



7 



8-6 



6-9 



10 : 4'9 or 5-9 



As we have seen previously, the skeleton of man and the 

 other Vertebrates is built after the same plan : a head, vertebral 

 column, shoulder girdle, ribs, pelvis and limbs (whose cardinal 

 number of four is never exceeded). Both in mammals and 

 man the pelvis is rigid, while the shoulder girdle is loosely 

 articulated with the spine. In the standing position in quadru- 

 peds, the centre of gravity of the body is maintained by muscular 

 effort vertically above the supporting surface enclosed by the 

 four outspread feet (the foot quadrilateral) (see Fig. 100). The 

 usual attitude of the head brings the centre of gravity slightly 

 nearer to the forequarters than to the hind ; this may be pro- 

 portionately represented as the weight of the fore and hind- 

 quarters. 



Horses 



Oxen .... 



Pigs . 



Dogs . 



In birds the centre of gravity 

 falls in front of the hip-joint 

 (Fig. 101), the ability to stand 

 on one leg being secured by 

 special mechanism of the joints 

 of the leg and foot. 



Weber states that the centre 

 of gravity of man lies in the 

 true pelvis, close in front of the 

 promontory of the sacrum; in standing it falls through the 

 soles of the feet so that the fixed arches of the two feet form 

 the only support to the body; if the toes are turned out at 

 right angles when the heels are together, the supporting surface 

 is greatly increased. In order to stand still more firmly, the 

 legs are spread apart as in sailors or fencers. When a man 

 wishes to stand on one leg, the centre of gravity has to be 

 inclined to the supported side, either by bending the back- 

 bone laterally, or by pushing over the opposite side of the 

 pelvis. 



The superiority of the erect position in man over that of 

 other mammals (apes, bears) consists in his ability to completely 

 straighten the thigh. In other animals the knee remains bent 



13 



FIG. 101. How the bird stands. 



