270 



THE HUMAN MACHINE 



Bad posture iu the 

 schoolroom may 

 cause permanent 

 injury to the spine. 



erect carriage, not only because of the better 

 state of health which comes with it, but also 

 because one's self-respect demands that each 

 one of us makes the best of the gifts that 

 nature has given us. An erect head, straight 

 shoulders, and elastic carriage go far toward 

 making their owner both liked and respected. 



Other Body Structures. — In spaces between 

 the muscles are found various other structures, 

 — blood vessels, which carry blood to and from 

 the great pumping station, the heart, and 

 thence to all parts of the body ; connective 

 tissue, which holds groups of muscle or other 

 cells together ; fat cells, scattered in various parts of the body ; 

 various gland cells, which manufacture enzymes ; and the cells 

 of the nervous system, which aid in directing the body parts. 



Body Cavity. — Within the body is a cavity, which in life is 

 almost completely filled with various organs. A thin wall of 

 muscle called the diaphragm divides the body cavity into two 

 unequal spaces. In the upper space are found the heart and lungs, 

 in the lower, the digestive tract with its glands, the liver, kidneys, 

 and other structures (see page 267). 



Digestion, Absorption, and Excretion. — Running through the 

 body is a food tube in which undigested food is placed and from 

 which digested or liquid food is absorbed into the blood so that the 

 cells of the various organs which do the work may receive food. 

 Emptying into this food tube are various groups of gland cells, 

 which pour digestive fluids over the solid foods, thus aiding in 

 changing them to liquids. Solid wastes are passed out through 

 the posterior end of the food tube, while liquid wastes are excreted 

 by means of glands called kidneys. 



Work done by Cells. — Food, prepared in the digestive tract, 

 and oxygen from the lungs are taken by the blood to the cells. 

 Bathed in liquid food, the cells do their work; they promote 

 the oxidization of food and the exchange of carbon dioxide for 

 oxygen in the blood, while other wastes of the cells are given off, 

 to pass eventually through the kidneys and out of the body. 



