116 THE HUMAN MACHINE 



health which comes with it, but also because self-respect demands 

 that we make 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; 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 various parts of the body. 



Body Cavity. — Within the cover of skin, bone, and muscle 

 is a cavity filled with various organs. A thin wall of muscle 

 called the diaphragm (df d-fram) 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, 

 the kidneys, and other structures (see page 111). 



Digestion and Excretion. — The mouth cavity leads into a 

 food tube in which undigested food is placed and from which di- 

 gested or liquid food is absorbed into the blood to be carried to 

 the cells of the various organs which do the work. 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 a soluble form. Solid wastes are passed out through 

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

 eventually excreted by means of the skin and of organs called 

 kidneys. 



The Nervous System. — This complex machine is much more 

 than a mechanical engine. It is self-directed. All its functions 

 are either directly or indirectly under its control. Not only are 

 animals able to receive outside stimuli through certain parts called 

 sense organs, but they react to them, and there is internal coordi- 

 nation and control as well. The complicated machine does its 

 work automatically; the heart beats, the glands secrete, the 

 chest rises and falls without any conscious direction on our part. 

 The nervous system gives sensation, it gives internal control 

 and coordination. In man it does more. It also gives him con- 

 trol over his conscious activities. He is able to make a selection 



