54 8 A MANUAL OF PHYSIOLOGY 



example to a very considerable extent, and yet recover its original 

 length when the stretching force ceases to act. 



The extensibility of a body is measured by the ratio of the increase 

 of length, produced by unit stretching force per unit of area of the 

 cross-section, to the original length of a uniform rod of the substance. 



If e is the extensibility, e = ^ where / is the increase of length, 



L the original length, s the cross-section, and F the stretching force. 

 Suppose we wish to compare the extensibility of two substances. 

 Let A and B be strips or rods of the substances, the length of A 

 being 500 mm., that of B 1,000 mm. ; the cross-section of A. 100 

 sq. mm., of B, 200 sq. rnm. Let the elongation produced by a 

 weight of i kilo be 10 mm. in each, then the extensibility of A is 

 10 x 100 ro x 200 

 ^y = 2 ; and that of B is ~^^~^ = 2 > tnat 1S ' tne substances 



are equally extensible. Young's modulus of elasticity, or the co- 

 efficient of elasticity is the quotient of the deforming force acting on 

 unit area of the given body by the deformation produced (within the 



ff j r ff 



limits of elasticity). In the above example it is -J-y that is , 



the reciprocal of the extensibility e. For steel the coefficient of 



elasticity is very large, for muscle small. 

 Or, as we may otherwise express it, 

 living muscle within its limits of elasticity 

 is very extensible ; a small force per 

 unit area of cross-section of a prism of 

 it will produce a comparatively great 

 elongation. The extensibility, however, 

 diminishes continually with the elon- 

 gation, so that equal increments of 

 stretching force produce always less 

 and less extension. If, for instance, 



FIG. i6o.-CuR VES OF EXTEN, th ? sa ' torius or semi-membranosus of 

 SIBILITY. a frg De connected with a lever writing 



M, of mu'cie ; S, of an ordinary on a blackened surface, and weights 

 inorganic solid. increasing by equal amounts be succes- 



sively attached to it, the recording sur- 

 face being allowed to move the same distance after the addition of each 

 weight, a series of vertical lines, representing the amount of each 

 elongation, will be traced. When the lower ends of all the vertical 

 lines are joined, a smooth curve with the concavity upwards is 

 obtained (Fig. 160). This is a property common to living and dead 

 muscle and to other animal structures, such as arteries. Marey's 

 method, in which the weight is continuously increased from zero and 

 then continuously decreased to zero again by the flow of mercury 

 into and out of a vessel attached to the muscle, gives directly the 

 curve of extensibility. 



The elongation of a steel rod or other inorganic solid is propor- 

 tional within limits to the extending force per unit of cross-section ; 



