4 ANATOMY FOE NURSES. [Chap. I. 



varies greatly, and hence in some substances the molecules can 

 only with great difficulty be broken up into their component 

 atoms. Such substances are said to be "stable." On the 

 other hand, many substances are very easily decomposed, and 

 are known as " unstable " substances. Between these two 

 extremes there are substances possessing every degree of 

 stability. 



In protoplasm, or proteid (proteid being the name usually 

 employed by chemists), the molecule is composed of carbon, 

 hydrogen, nitrogen, oxygen, and sulphur, and is a highly com- 

 plex structure. It is also extremely unstable, and is very 

 sensitive to outside influences. The many vital phenomena 

 exhibited by protoplasm are due, in great part, to the chemi- 

 cal reactions of the atoms composing its molecules, and which 

 are rendered possible by the great instability of these mole- 

 cules. 



During the life of a cell its protoplasm is constantly under- 

 going changes, the chief of which may be enumerated as 

 follows : — 



(1) All protoplasm coming in contact with oxygen absorbs 

 it and combines with it. Whenever this combination takes 

 place, a certain amount of the protoplasm is burned or oxi- 

 dized, and as a result of this oxidation heat and other kinds of 

 energy are produced, and carbon dioxide evolved. 



(2) All protoplasm is able to take to itself, and eventually 

 convert into its own substance, certain materials (foods) that 

 are non-living; in this way the protoplasm may increase in 

 amount, or in other words the cell may grow. But if the 

 amount of protoplasm does not permanently increase, this is 

 due to the fact that just as much protoplasm is being broken 

 down by the process of oxidation, and removed from the cell, 

 as is added by the process of assimilation. Chemical changes 

 which involve the building up of living material within the 

 cell have received the general name of anabolic changes, or 

 anabolism ; those, on the other hand, which involve the break- 

 ing down of such material into other and simpler products, 

 are known as katabolic changes, or katabolism ; while the sum 

 of all the ana- and katabolic changes which are proceeding 

 within the cell are spoken of as the metabolism of a cell. 

 These chemical clianges are always more marked as the activ- 



