PHYSICAL PROPERTIES OF MUSCLE 735 



run down through the cell from the basal bodies (Figs. 241, 242). In 

 some worms and molluscs ciliated cells are supplied with nerve-fibres, 

 but this has not been demonstrated for the higher animals. 



SECTION II. PHYSICAL PROPERTIES AND STIMULATION OF MUSCLE. 



Since most of our knowledge of the general physiology of muscle 

 has been gained from striped muscle, in what follows we always 

 refer to ordinary skeletal muscle, unless it is otherwise stated. 

 The sartorius and the gastrocnemius are the classical objects for 

 experiments on striated muscle. For smooth muscle the adductor 

 muscle of Anodon, the fresh-water mussel, a ring cut from the middle 

 portion of the frog's stomach, the rabbit's ureter and uterus, and 

 the cat's bladder, have been most used. 



Physical Properties of Muscle Elasticity. All bodies may have their 

 shape or volume altered by the application of force. Some require a 

 large force, others a small force, to produce a sensible amount of dis- 

 tortion. The elasticity of a body is the property in virtue of which it 

 tends to recover its original form or bulk when these have been altered. 

 Liquids and gases have only elasticity of volume; solids have also 

 elasticity of form. Most solids recover perfectly, or almost perfectly, 

 from a slight deformation. The limits of distortion within which this 

 occurs are called the limits of elasticity, and they vary greatly for 

 different substances. Living muscle has very wide limits of elasticity; 

 it may be deformed stretched, for 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. 



Is 

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



L the original length, 5 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. mm. Let the elongation produced by a weight of i kilo 



be 10 mm. in each, then the extensibility of A is = 2 ; and that 



500 x i 



10 x 200 



of B is - = 2; that is, the substances are equally extensible. 



. 1,000 x i 



Young's modulus of elasticity, or the coefficient of elasticity, is the 

 quotient of the deforming force acting on unit area of the given body 

 by the deformation produced (within the limits of elasticity). In the 



"pj T f? 



above example it is -*-?- that is, -=, the reciprocal of the extensi- 

 bility 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 elonga- 

 tion. The extensibility, however, diminishes continually with the 

 elongation, so that equal increments of stretching force produce always 



