30 PHYSIOLOGY CHAP. 



Kanvier draws the same conclusions from the fact brought out 

 directly by his experiments, viz. that the anisotropous discs are 

 the only ones that change in form and diminish in volume during 

 the state of isomeric tetanisation. 



More recently Schafer (1891) and Hiirthle (1901-4) have 

 studied the microscopic variations in the muscle fibres during 

 contraction, by photography and cinematography. Schiii'er's 

 observation in particular, according to which minute canals, 

 parallel with one another, run in the anisotropous layer in the 

 direction of the fibres, is important. During contraction the 

 isotropous substance penetrates these canaliculi, which dilate so 

 that the muscular segment becomes wider and shorter. 



It is in any case certain that the transverse striation due to 

 the separation of the doubly refracting from the singly refracting 

 fibres is not indispensable to the contractility of the elements, 

 because the unstriated muscle cells are contractile although much 

 more sluggishly so than the striated fibres. Kanvier assumes in the 

 latter that the separation of the doubly refracting substance into 

 distinct masses facilitates and makes possible a quicker displace- 

 ment of the fluid from the surrounding parts into the contractile 

 layers. 



VI. We must next consider the phase of relaxation, in which 

 the shortened muscle elongates and describes a curve which 

 closely resembles the curve of contraction. The sole difference 

 between contraction and relaxation lies in the fact that the latter 

 is, generally speaking, more variable in its duration and rate of 

 drop towards the abscissa. 



Formerly the elongation of the contracted muscle was regarded 

 as a physiologically passive phenomenon, due to the cessation of 

 the process of contraction. Very few admitted that both the 

 shortening and the lengthening of the muscle were due to 

 converse physiological processes : yet this theory of the con- 

 tractive and expansive activity of skeletal muscle, which we have 

 maintained since 1871, agrees with the corresponding theory of 

 the properties of amoeboid protoplasm, cardiac muscle, and the 

 musculature of the vessels and gut, which was discussed at length 

 in Vol. I. 



The length of any skeletal muscle in the resting state is not 

 constant, but varies under different intrinsic and extrinsic 

 conditions. 



When any muscle or the tendon by which it is attached to 

 the bone is divided in the living animal, the two segments draw 

 apart or retract, as though the muscle were normally in elastic 

 tension and the distance from the points of its insertion were 

 greater than the natural length. 



Cut muscles also retract after death, so that the tension of 

 normal skeletal muscle is partly an effect of the elasticity of the 



