226 



FIRST YEAR SCIENCE 



passes down through the windpipe into the lungs. In order 

 to keep out dust and germs, the opening of the nose is 

 supplied with a large number of hairs projecting from the 

 mucous membrane which lines the whole nasal chamber. 

 These hairs and the secretion from the membrane catch and 



hold most of the harm- 

 i . I 



ful particles. At the 

 back of the mouth the 

 windpipe and the throat 

 come together. 



When food is being 

 swallowed, the passage 

 into the windpipe must 

 be closed, and this is 

 done by the little valve- 

 like epiglottis. If, in 

 swallowing, the epi- 

 glottis is not able to 

 close quickly enough, 

 something may pass into the windpipe and cause choking. 

 The windpipe, at the upper part of the chest, branches into 

 two parts, one branch going to each of the lungs. 



The lungs fill the upper part of the chest and enfold 

 the heart. In them the air tubes divide again and again, 

 forming a vast network of tubes which grow smaller and 

 smaller until they end in little air sacks. Interlacing with 

 these air tubes are veins and arteries which carry the blood. 

 The tiniest parts into which the blood vessels are divided, 

 the capillaries, form close networks within the linings of 

 the air sacks. The air and blood are thus separated by an 

 exceedingly thin animal tissue, which allows an exchange 

 of soluble materials. Thus the blood is able to take-up the 

 oxygen needed and to rid itself of the carbon dioxide and 

 other waste products which it has accumulated. 



THE LUNGS. 



They are here pulled aside to show the 

 heart. 



