MUSCLE 547 



of cells do not move at the same time ; the motion spreads 

 from cell to tell in a regular wave. The energy of ciliary 

 motion may be considerable, although far inferior to that 

 of muscular contraction. The work which cilia are capable 

 of performing can be calculated by removing the membrane, 

 fixing it on a plate of glass, cilia outwards, putting weights 

 on the glass plate, and allowing the cilia, like an immense 

 number of feet, to carry it up an inclined plane. Bowditch 

 found in this way that the cilia on a square centimetre of 

 mucous membrane did nearly 7 gramme-millimetres of work 

 per minute (equal to the raising of 7 grammes to a height of 

 a millimetre). 



Since the cilia in the respiratory tract all lash upwards, 

 they must play an important part in carrying up foreign 

 particles taken in with the air, and the mucus in which they 

 are entangled, as well as pathological products. Engelmann 

 found that the energy of ciliary motion increases as the 

 temperature is raised up to about 40 C., after which it 

 diminishes quickly. Overheating causes cilia to come to 

 rest, but if the temperature has not been too high, and has 

 not acted too long, they recover on cooling. 



Muscle. Nearly all our knowledge of the physiology of 

 muscle has been gained either from striped skeletal muscle 

 or from the muscle of the heart, and chiefly from the former. 

 Of non-striped muscle we know comparatively little except 

 by inference, owing to the difficulty of obtaining it in suffi- 

 cient quantity and in suitable preparations for experiments. 

 In what follows we always refer to ordinary skeletal muscle, 

 unless it is otherwise stated. 



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 



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