130 EXTENSIBILITY, ELASTICITY, AND WORK OF MUSCLE [CH. XI. 



at certain temperatures near the freezing-point, and under the 

 influence of certain weights, actual elongation may occur when the 

 temperature is raised. 



Muscular Tonus. 



In the living animal, muscles are more or less stretched, but 

 never taut between their two attachments. They are in a state of 

 tonicity or tonus, and when divided they contract and the two parts 

 separate. Thus a muscle, even at rest, is in a favourable condition 

 to contract without losing time or energy in taking in slack. 



Muscular tonus is under the control of the nervous system (on 

 the reflex character of this control, see later, under Tendon Eeflexes) ; 

 the muscles lengthen when their nerves are divided, or when they 

 are rendered physiologically nerveless by curare. Besides the nervous 

 system, the state of muscular nutrition dependent on a due supply 

 of healthy blood must also be reckoned as important in maintaining 

 muscular tonus. 



Work of Muscle. 



The question of muscular work is intimately associated with that 

 of elasticity. In a technical sense, work (W) is the product of the 

 load (/) and the height (h) to which it is raised. W = l x li. 



Thus in fig. 157, when the muscle is unloaded the work done is 

 nil: W = BCxO = 0. When the load is 250, again the work done 

 is nil, because then h = 0. With the load 50, W = B' C' x 50. 



If the height is measured in feet and the load in pounds, work is 

 expressed in terms of foot-pounds. If the height is measured in 



FIG. 158. Diagram to show the mode of measuring muscle work. (M'Kendrick.) 



millimetres or metres, and the load in grammes, the work is expressed 

 in gramme-millimetres or gramme-metres respectively. 



This may be shown diagrammatically by marking on a horizontal 

 base line or abscissa, distances proportionate to different weights, 

 and vertical lines (ordinates) drawn through these represent the 

 height to which they are lifted (see fig. 158). 



In the diagram (fig. 158) the figures along the base line represent 

 grammes, and the figures along the vertical line represent milli- 

 metres. . The work done as indicated by the first line is 10x5 = 50 



