296 PHYSIOLOGY OF MUSCLES AND NERVES. 



shown by a metliod first employed by Descartes, -vvhicli it is 

 our present object to explain. 



The amounts treated may be of the most varied kinds : 

 numbers, weights, degrees of warmth, the number of births 

 or deaths, and so on. In all cases the amount may be diagram- 

 matically shown by the length of a line. If a line of a cer- 

 tain length represents any given amount, then double this 

 amount is represented by a line twice the length of the 

 former. It does not matter what is the standard selected ; 

 but when once selected it must not be varied in the same 

 representation. Two lines are drawn at right angles to 

 each other ; from the point of section B (fig. 69) the lengths 

 which are to represent the values of one series (in our case, 

 the weights attached to the muscle) are measured off on the 

 e 

 ^ f 



Jx 



Fig. 70. Diagram of positive and negative values. 



horizontal line. From each of the points thus obtained, cZ', h" , 

 d", d'", a line is drawn at right angles to the first, care being 

 taken to make its length express the expansion corresponding 

 with each weight respectively. This gives the lines d' B\ 

 h" B", d" B'", d'" j5^^'. By connecting these points we 

 obtain the curve BB' B" B'" B''' x" , which at a glance 

 shows the relation between the weight and the expansion. 

 In exactly the same way the curve h h' h" h'" B'" y is pro- 

 jected, and this represents the expansion of the active muscle 

 by the corresponding weights. 



In many cases it is required to represent values of oppo- 

 site kinds. If, for example (fig. 70), the wire a 6 is tra- 

 versed by an electric current, then one half assumes positive 

 tension, the other negative tension. To express this, the 



