RHYTHM IN TISSUES i;}7 



makes a spiral movement through the air. This circular 

 movement which becomes a spiral is caused by a rhythmic 

 change in the growing cells. Certain of them will grow lastcr 

 than the others and then assume their normal rale, lichiml 

 them another set of cells will now start growing faster, and 

 this is continued until the circle has been completed. 



The growth of plants is not continuous, but is perpetually 

 interrupted in a rhythmical manner. The growth of the 

 Spirogyra proceeds by fits and starts; there is a period of 

 activity followed by a period of rest, the intervals extending 

 over some minutes, usually about twenty. Very refined 

 measurements have shown that plant growth takes place by 

 minute but perfectly rhythmical pulsations at intervals of 

 a few seconds of time. The stalk of the crocus, for instance, 

 grows by little jerks, each with an amplitude of 0-002 mm. 

 at periods of twenty seconds or so; and after each little 

 increment there is a partial recoil. 



In many forms of movement there is order in time as well 

 as in direction. We have seen this in the case of cilia, and we 

 find it again when waves travel along muscular fibres or from 

 one fibre to another. The muscular movement of the stomach 

 and the alimentary canal which forces the food along is 

 termed peristalsis. A constriction takes place in one part of 

 the tubular intestine and this constriction passes rhythmically 

 downwards, and this it does independently of the nervous 

 system. This contraction of the walls of the alimentary canal 

 which is propagated from muscle fibre to muscle fibre takes 

 place at the rate of about 5 cm. per second. 



A further rhythm connected with digestion occm-s in the 

 cells of the pancreas, the liver and the intestinal glands. In 

 a fasting dog they secrete their digestive juices for a period 

 of twenty to thirty minutes each hour, and then have a rest. 

 Another instance of involuntary muscles which possess the 

 property of rhythmic action is shown in the lower end of the 

 frog's or tortoise's heart. These can be removed from the 

 body and kept pulsating under certain conditions for many 

 days, removed from all nervous stimulus. The heart of tlie 

 embryo chick begins to beat before any nerves have grown 

 into it. The theory that muscles can contract without being 



