CHARACTERISTICS OF ANIMALS 25 



capillarity, or the power that makes water stand higher at the 

 edges of a glass than in the center, exerts a profound influence 

 on the nature of the chemical reactions that take place. Second, 

 surface tension, the force that permits water to " heap up " slightly- 

 above the top of a glass so that it is more than level full, or 

 that lets a dry needle float on the surface of water, keeps the 

 compounds of different density separate, so that each may be 

 almost as distinct from the other as though it occupied a dif- 

 ferent test-tube. Third, osmosis, the force that permits liquids 

 of different densities to interchange through bladder membranes, 

 allows some interchange of the products of one reaction through 

 the different compounds of the protoplasm. 



Living matter (protoplasm) is essentially something that per- 

 forms a function. It does not remain in a state of equilibrium, 

 but is continually either storing or liberating energy under fixed 

 conditions and may be thought of as a mechanism in which 

 chemical processes of a special nature are continuously going on. 

 These processes are called metabolism and are of two kinds, viz. : 

 anabolism and katabolism. Anabolism, or the building-up 

 process, is characteristic of plants. In the presence of sunlight 

 the green leaf takes carbonic acid gas from the air, and water 

 with mineral elements from the soil, and evolves a more complex 

 substance from them that possesses a high degree of energy. Our 

 animals benefit from this and find here materials to support 

 their breaking-down process, or katabolism. The energy thus 

 stored by the plant is released to the animal for his use. The 

 combination of these metabolic processes is one of the safest 

 methods of distinguishing simple living matter from non-living. 

 . From a physiologic standpoint we may group processes that 

 are constantly going on in living matter under the following 

 ten headings: 



1. Ingestion is the power of taking food particles into the pro- 

 toplasm. In the simplest cases it probably represents merely 

 the flowing of the protoplasm around the food. 



2. Secretion is the response of the protoplasm to these food 

 particles. It corresponds in a primitive way to the production 

 of the saliva in the mouth and the gastric juice in the stomach. 

 It is simply the production of certain juices, activators, enzymes, 

 or hormones, as they are variously called, that have a chemical 

 action on the food. Each kind of body cell has its own particu- 



Digitized by Microsoft® 



