THE MO FEME NTS AND POSITIONS OF THE BODY. 231 



+ z 



is that through which pass the lines zz and xx. The lines in the figure 

 are each the lines of intersection of two planes ; the point i> s the point 

 of intersection of the three planes. If now we wish to define the posi- 

 tion of the point p in reference to these three co-ordinate planes, 

 we can drop a line vertical to the horizontal plane, cutting it at a. 

 ■pa is the distance of the point p from the horizontal plane xx and yy. 



If, now, we draw a line ah perpendicular to the vertical plane zz and yy, 

 this will give the distance of p from that plane. The distance bo gives 

 finally the distance of p from the second 

 vertical plane zz and xx. It is neces- 

 sary further to notice the signs of pa, 

 ah, and ob, which, in the case under 

 consideration, are all positive. Were 

 p below the floor, pa would he nega- 

 tive ; for the movement of p around 

 would he in the direction of the hands 

 of a watch. The method of describing 

 the position of the body in reference to 

 three co-ordinate planes has been greatly 

 used by those who have studied the 

 animal mechanisms, and by no one more 

 elaborately than by Braune and Fischer. 



In their investigation into the centre 

 of gravity of the human body, they 

 start with an erect position, in which 

 the chief joints of the body, and the 

 centres of gravity of the trunk and 

 limbs, are all in one frontal plane. In 

 Fig. 122, B, simplified from their original 

 figure, this plane is vertical to the paper, 

 and runs through the line zz. In 

 Fig. 122, A, the plane is seen "from the Fig. 121.— Shows the method of determin 



ing the position of the point (p) in 

 reference to three co-ordinate planes 

 intersecting in the point O. 



front," and is mapped out in equal 



squares, giving distances along the line 



yy from the centre of the plane zz, 



these being reckoned as negative to the right-hand side, and positive to the 



left-hand side. Along this plane can also be measured distances from the 



ground plane in a vertical direction. 



Cutting this plane through the line zz, and running in the mesial plane of 

 the body, is the sagittal plane, seen from the left in Fig. 122, B. This is also 

 divided into equal squares ; and distances forwards from the axis zz of inter- 

 section of the two planes are counted as positive, while distances backwards 

 are counted as negative, both passing in the direction xx of the figure. 



Co-ordinates on X, Y, and Z Planes. 



In describing the position of any point of the body, say the atlanto- 

 Gccipital joint, this will, by construction, 1><> in the frontal plane, and there- 

 fore its deviation in the xx direction will he 0. As it is also in the mesial 



