G12 



COMPARATIVE PHYSIOLOGY. 



similar and antagonistic muscles so co-ordinated that the line 

 of gravity falls within the area of the feet. An unconscious 

 animal falls, which is itself an evidence of the truth of the 

 above remarks. 



The following statements in regard to the direction of the 

 line of gravity in man may prove useful : 1. That for the head 

 falls in front of the occipital articulation, as exemplified by the 

 nodding of the head in a drowsy person occupying the sitting 

 attitudes. That for the head and trunk together passes behind 

 a line joining the centers of the two hip-joints, hence the uncor- 

 rected tendency of the erect body of man is to fall backward. 

 3. That for the head, trunk, and thighs falls behind the knee- 

 joints somewhat, which would also favor falling backward 

 (bending of the knees). 4. The line of gravity of the whole 

 body passes in front of a line joining the two ankle-joints, so 



Fig. 453.— Shows the simaltaneous positions of both legs during a step, divided into 

 four groups" (after Weber). First group (A), 4 to 7, gives the different positions 

 which the legs simultaneously assume while both are on the ground; second group 

 (B), 8 to 11, shows the various positions of both legs at the time when the poste- 

 rior leg is elevated from the ground, but behind the supported one; third group 

 ( C), IS to 14, shows the positions which the legs assume when the swinging leg 

 overtakes the standing one; and the fourth group (D), 1 to 3, the positions during 

 the time when the swmging leg is propelled in advance of the resting one. The 

 letters a, 6, and c indicate the angles formed by the bones of the right leg when 

 engaged in making a step; the letters m, % and o, the positions assumed by the 

 right foot when the trunk is rolling over it; g, shows the rotating forward of the 

 trunk upon the left foot (/) as an axis; h, shows the rotating forward of the left 

 leg and foot upon the trunk (a) as an axis. 



that the body would tend, but for the contraction of the mus- 

 cles of the calves of the legs, to fall forward. 



Taking these different facts into consideration explains the 



