140 



RHYTHM 



healthy adult human being the heart contracts about 72 times 

 a minute, but its frequency is interfered with by many factors — 

 exercise, time of day, atmospheric pressure, temperature, 

 food and drink. There is a steady diminution of the number of 

 contractions as age advances. Just after birth the contractions 

 recur at the rate of 140-130 a minute. These drop in the 

 second year to 115-110 and in the seventh year to 90-85. 

 The contractions of a boy of 14 years will be 85-80 and in 

 adult age 80-70. This, however, drops to 70-60 in old age. 



Other very important organs which contract and expand 

 rhythmically are the walls of the chest. The number of 

 respirations at rest in a normal human being is about 14 to 

 18 per minute, 1 to 4 or 5 in proportion to the heart beats. 



Fig. 50, lulus terrestris, sometimes called the "Wire-worm." From Koch. 

 Magnified about 3 J. 1. Antemiae. 2. Eyes. 3. Legs. 4. Pores for the escape 

 of the excretion of the stink-glands. 



If the heart beat be quickened, the rhythm of the lungs is 

 accelerated. Like the heart, the frequency of the respiratory 

 movements is greatest in infancy and childhood. 



If you turn up a stone lying in a damp wood or dig a little 

 in a rich soil you will very probably come across a little black 

 millipede, known as lulus. It is unfortunate that this is known 

 to the farmers as a wire- worm, because they call the larva of 

 a beetle (Elater lineatus) by the same name. lulus does a 

 considerable amount of damage by nibbling the tender roots 

 of growing plants. Like other millipedes, it has an extra- 

 ordinary number of legs. In the group to which millipedes 

 belong, Myriapoda, the number of legs sometimes amounts 

 to 150 — 200. When the lulus is crawling along you can see 

 a succession of waves passing along the legs on each side of 

 the body in a definite rhythm, just as one sees waves pass 

 over a cornfield when the wind is blowing. It is not easy to 



