TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 39 



machine; it is without power to manufacture energy, and must 

 receive whatever energy it obtains from without. The ultimate 

 source of the Body's energy is chemical, being received in the com- 

 plex substances which serve as food. This energy is made avail- 

 able for the use of the body chiefly through the process of oxida- 

 tion. Every living cell in the body must share in this process, for 

 the energy manifestations of the body as a whole are simply the 

 sum-total of those of its component cells. 



The systems which are concerned with the maintenance of ac- 

 tivity have, then, the task of furnishing to each cell of the body 

 oxidizable substance and oxygen; they must provide for making 

 good the wear and tear of the cells themselves; and they must 

 remove the waste materials which are formed in connection with 

 the chemical activities of the cells and which would interfere with 

 their proper working if allowed to accumulate. 



The Circulatory System consists of the heart and blood-vessels. 

 It serves to distribute to all the parts of the Body supplies of 

 oxidizable material, of repair material, and of oxygen, and to re- 

 move these from the accumulated waste products. These func- 

 tions are accomplished through the agency of a circulating me- 

 dium, the blood. 



The Respiratory System consists of the lungs, the bronchial 

 tubes, and the trachea, together with the respiratory muscles. Its 

 function is to bring the outside air to a region where the circulating 

 medium can take up abundant supplies of oxygen, and where it 

 can get rid of those waste products which are in gaseous form. 



The Digestive System consists of the alimentary canal and cer- 

 tain associated glands (salivary, liver, pancreas) . It serves to bring 

 the various materials that are taken as food into the forms best 

 adapted for use as repair materials or as oxidizable substance; 

 when it has so prepared them it turns them over to the circulating 

 medium for distribution. 



The Excretory System consists of the kidney and bladder with 

 their connecting tubes, the liver, and the skin. It serves to with- 

 draw from the circulating medium and to eliminate from the body 

 those waste products which are in liquid form. 



Through these systems provision is made for the activities of the 

 individual cells. These activities are many and complex. They 

 include oxidative processes, processes involving waste and repair, 



