i 9 4 



THE HUMAN SPECIES 



when standing upright, and a constant muscular effort is re- 

 quired to prevent its further flexion. 



Man, however, can completely extend his knees, and his 

 body-weight has rather the tendency to over-extend his joints, 

 a movement which is mechanically opposed, not by constant 

 muscular activity, but by the tendinous lateral and popliteal 

 ligaments. When sitting, the weight of the upper part of the 

 body rests on the tubera ischii, as a rocking-horse on its rockers. 



The centre of gravity of the upper 

 portion of the body lies in front of the 

 tenth dorsal vertebra, and the body is 

 maintained in its position of unstable 

 equilibrium by a muscular effort which 

 fixes the spine, so that the sitting posture 

 becomes very fatiguing unless the back 

 is supported. Every movement of the 

 human or animal body is attended with 

 a change in the centre of gravity, and an 

 alteration in the position of the joints in- 

 volved in the movement ; the mechanism 

 of the joints is controlled by the same 

 general rules, both in man and beast. But 

 in certain details characteristics peculiar 

 to man can be observed. 



The articulation between the trapezium 

 and the os primi metacarpi is a saddle 

 joint allowing movement in two planes, 

 so that in addition to flexion and exten- 

 sion, not only lateral movement but even 

 a limited degree of rotation are possible. 

 The ankle joint between the tibia and os 

 calcis is in man a simple hinge joint, while in animals it is 

 a screw-ginglymus, the heel forming a segment of a screw, the 

 articular surface of the tibia being a segment of a corresponding 

 socket. 



The human knee is a spiral joint, the antero-posterior curve 

 of the condyles of the femur presenting in sagittal section a 

 spiral outline. The articular surface of the lower jaw is similar 

 in man and the other omnivora, the head is oval with its long 



FIG. 102. Man standing 

 naturally. (Munk- 

 Schultz.) 



