Chap. Ill] CELLS 23 



place, a certain amount of the protoplasm is burned or oxidized, 

 and as a result of this oxidation heat and other kinds of energy 

 are produced, and carbon dioxide, which is a waste product, is 

 evolved. Thus it will be seen that the real purpose of respiration 

 is to furnish oxygen to each individual cell, and to take from the 

 cell the carbon dioxide which it does not need. 



(2) Metabolism. — Each cell is able to take to itself, and eventu- 

 ally 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. The amount of protoplasm 

 is not permanently increased, because 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 ; on the other hand, those which involve the breaking 

 down of such material into other and simpler products are known 

 as katabolic changes, or katabolism, while the sum of all the ana- 

 bolic and katabolic changes which are proceeding within the cell 

 is spoken of as the metabolism of a cell. These chemical changes 

 are always more marked as the activity of the cell is promoted by 

 warmth, electrical, or other stimulation, the action of certain drugs, 

 etc. 



(3) Amosboid movement. — The most obvious physical changes 

 that can sometimes be seen in living protoplasm, by the aid of 

 the microscope, are those which are termed "amoeboid." This 

 term is derived from the amoeba, a single-celled organism which 

 has long been observed to exhibit spontaneous changes of form, 

 accompanied by a flowing of its soft semi-fluid substance. By 

 virtue of this property, the cells can move from one place to 

 another. If one of these cells be observed under a high-power 

 lens of the microscope, it will be seen gradually to protrude 

 a portion of its protoplasm ; this protrusion extends itself, and 

 the main part or body of the cell passes by degrees into the 

 elongated protrusion. By a repetition of this process, the cell 

 may glide slowly away from its original situation and move 

 bodily along the field of the microscope, so that an actual 

 locomotion takes place. When the surface of these free cells 

 comes in contact with any foreign particles, the protoplasm, by 



