METABOLISM 



35 



or by means of a circulatory system, generally the blood circulatory 

 system. 



51. Inspiration. — Oxygen, as well as food, is constantly needed by 

 the body. Its entrance into the body is termed inspiration. This may 

 occur through all points on the surface of the body or may occur only 

 through certain particular organs set aside for the purpose, such as lungs 

 or gills. Upon entrance into the body oxygen is circulated in the same 

 manner as food and taken up by the tissues as needed. This passage of 

 oxygen into the tissues is termed internal inspiration; its entrance into 

 the body, external inspiration (Fig. 9). 



Fig. 9. — Diagram to illustrate external and internal respiration. 



52. Assimilation. — The food, having been brought to the point in 

 the body where it is to be used, is taken up by the protoplasm and more 

 or less intimately incorporated into the living mass, becoming, at least 

 for the time, a part of the organization. This process of addition of new 

 material to the existing material of the body is termed assimilation. This 

 material, no longer food but a part of the protoplasm, may be soon used 

 or it may remain for a greater or less length of time as a part of the cell 

 before actually becoming involved in chemical changes. 



53. Dissimilation. — Sooner or later chemical changes occur which 

 collectively are called dissimilation, as a result of which protoplasm and 

 the more complex food substances associated with it are broken down 

 into simpler substances. Associated with these chemical changes is a 

 transformation of part of the potential energy represented by these 

 substances into kinetic energy, which appears mostly in the form of heat 

 or movement. 



54. Secretion. — If the substances produced in dissimilation can be 

 utiUzed in any way by the body as a whole, they are termed secretions, 

 and the process involved in their passing out of the cell which produces 

 them is termed secretion. These may be passed out upon the surface 

 of the body, into any cavity in the body, or into the blood and body 

 fluids. Examples of such substances are the tears, which when poured 



