630 A MANUAL OF PHYSIOLOGY 



observed in certain low forms of animals that cilia which have 

 been broken off from the cell are still able to contract when a 

 small portion of the substance of the cell-body at the point where the 

 cilium is attached to the cell, the so-called basal piece, or basal body 

 (Fig. 218), has come off along with them. In other forms isolated cilia 

 can contract in the absence of anything corresponding to the basal 

 piece. It cannot, therefore, be said that continuity with the basal 

 piece is absolutely necessary. Nor is it known what significance for 

 the ciliary movements is possessed by the long fibrillse, called the 

 ' roots of the cilia,' which in some animals run down through the cell 

 from the basal bodies (Figs. 217, 218). The theory has been put for- 

 ward by Schafer that the cilia are hollow processes of the cell-body, 

 and that their contraction is caused by the passage of liquid into 

 them from the cell. He believes that the direction of movement is 

 determined by the investing membrane of the cilium being thickened 

 (or less extensible) along one side or in a spiral line. In some worms 

 and molluscs ciliated cells are supplied with nerve-fibres, but this 

 has not been demonstrated for the higher animals. 



Muscle. Since most of our knowledge of the general physio- 

 logy 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 distortion. 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. 



If e is the extensibility, e^-^ t where I is the increase of length, 



JL/.T 



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 



10 X I0 =2 ; and that of B is IOX2 =2 ; that is, the substances 



500 X I 1,000 X T 



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



