ELASTICITY OF MUSCLES. 23 



proportionate to the length of the body extended, and 

 to the amount of the extending lueights ; and that it 

 is also proportionate in inverse ratio to the diameter 

 of the extended body. This is called the law of elas- 

 ticity, of Hook and S'Grravesande. In order, however, 

 to find the tension of a particular body, another factor 

 connected with the nature of the body itself must be 

 known ; for, under otherwise equal conditions, the ten- 

 sion, for instance, of steel, as found by actual experiment, 

 differs from that of glass, and that of the latter from 

 that of lead, and so on. In order, therefore, to be able 

 to calculate the tension in the case of all bodies, the 

 tension, experimentally found, must be reduced to the 

 units of length and diameter of the weighted bodies, 

 and to units of the weight applied. This gives a figure 

 which expresses the tension of a body of a given nature 

 of one millimetre in length, and with a cross-section 

 measuring one sc^uare centimetre when supporting a 

 weight of one kilogramme. This result, which is con- 

 stant in the case of every substance, whether it be steel, 

 glass, or aught else, is the co-efficient of elasticity of 

 that substance. 



6, Similar researches have been made in the case 

 of organic bodies also, such as caoutchouc, silk, muscle, 

 &c., and in so doing certain peculiarities have been 

 observed which are of course of great importance to us. 

 In the first place, all these bodies — which we may also 

 call soft, to distinguish them from those rigid bodies of 

 which, up to the present, we have been speaking — ex- 

 hibit a much greater extensibility. That is to say, soft, 

 organic bodies are capable of far greater extension than 

 are rigid, inorganic bodies of equal length and diameter, 

 and under the application of equal weight. But the 



