136 EXTENSIBILITY AND WORK OF MUSCLE. [CH. xi. 



traction can be explained in this way ; they have supposed that 

 the heat formed in muscular contraction acts like warmth as 

 applied to india-rubber. This view is, however, incorrect. It is 

 much more probable that there is no causal relationship between 

 the temperature-change and the extensibility-change which occur 

 when muscle contracts ; both are simultaneously produced by a 

 common cause, called a stimulus. 



Moreover, the influence of heat on muscle is by no means the 

 same as that on india-rubber. This influence is not invariable, 

 and 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 

 Reflexes, Chapter XLVIL); 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 (1) and the height (A) to which it is raised. W = I x h. 



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 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 hori- 

 zontal base line or abscissa, distances proportionate to different 

 weights, and vertical lines (ordinates) drawn through these repre- 

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



