29G PHYSIOLOGY OF MUSCLES AND NERVES. 



shown by a method first employe. 1 l>y 1 >escartes, which it is 

 our present object to explain. 



The amounts treated may be of the most varied kinds : 

 numbers, weights, decrees of \vunnth, the number of births 

 or deaths, and soon. In all rases the amount may bediai;ram- 

 matically sho\vn )>y 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 j 

 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 CMS-, 

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

 c 



^^ 



A 



TlG. 70. DlAGHA.M 01'- POSITIVE AM) .NKGATIVI. VAI.1I.S. 



horizontal line. From each of the points thus obtained, tl', b", 

 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 </ /.''. 

 I" B", d" B'", d" 1 7; iv . I'.y connect in- those points we 

 obtain tho curve Jill' It" B'" Jf* x", which at a glance 

 shows the relation h. 'tween the weight and the expansion. 

 In exactly the same \\ay the curve b b' b" b'" J> v // is pro 

 jecied, and this repn sents the expansion of the active muscle 

 by the corresponding weights. 



In many caSOB it is reijuired to represent values of oppo- 

 site kinds. Jf, for example (tig. 7<M, the wire a b is bra- 

 \er-rd by an electric current, then one half assumes jnjsitive 

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



