138 RHYTHM 



stimulated by nerves is known as the myogenic theory. The 

 blood capillaries show a rhythmic constriction and dilatation, 

 and still another example of rhythmic contraction of vessels 

 is found in the wings of bats. Plere the blood vessels have 

 minute valves which permit the blood to flow only in one 

 direction, and it is driven along by the vessels pulsating at 

 regular intervals. 



The rings which one finds in so many natural objects in- 

 dicate periodicity of growth. We have referred to the rings 

 shown by a section of a tree. The concentric spheres which 

 build up the starch-grains which form so permanent a feature 

 in plant life are believed to indicate a diurnal and nocturnal 

 periodicity of activity and rest, {^^^'"V^HH^^i^P'^H 

 although this may not be the By* ^^^^^ H 



full explanation. Both the scales Wi: Wj ■ 



of fishes, their vertebrae, and f T *■ 



the curious bone in the ear | |1| I 



termed the o^o/if^^ have marked i; ;■": ■ 



concentric rings. The subject 1^:^ -^mj^. , "J 



is a debated one, but many be- H^ ^^^Bk M^^k 



lieve, and indeed it seems fairly HHjJhHg^l^^^^^^^flJH 

 certain in the scales of many pjc. 47. Otoliths of Plaice, showing 

 fishes, that these rings indicate four zones or "age-rings." After 

 an annual growth comparable Wallace. 



with the annual rings of the higher plants. If you cut a 

 pearl in half and look at the flat surface you will see a 

 series of concentric rings. The cut passes through a series 

 of concentric spheres. This concentricity indicates a periodicity 

 of growth due to the deposit of a mineral, aragonite, of which 

 pearls are formed, taking place at intervals. The shells of 

 molluscs also show concentric lines of growth. 



Rhythm in Organs 



Many organs of the body act rhythmically. A notable 

 example of this is the heart in man. The duty of the heart 

 is to pump the blood round the body carrying with it oxygen 

 and food to be distributed to the general cells of the body, 

 and bringing back on its return carbon dioxide to be dis- 

 charged through the lungs, and certain excreta to be dis- 

 charged by the kidneys and sweat-glands. In a normal 



