24 THE HORSE IN HEALTH AND DISEASE 



density separate, so that each may be almost as distinct from the 

 other as though it occupied a different test-tube. Third, osmosis, 

 the force that permits Hquids of different densities to interchange 

 through bladder membranes, allows some interchange of the prod- 

 ucts of one reaction through the different compounds of the pro- 

 toplasm. 



Living matter (protoplasm) is essentially something that per- 

 forms a function. Energy is either being stored or hberated. 

 This process is called metabolism and is divided into anabolism and 

 katabolism. Anabolism, or the building-up process, is character- 

 istic 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 proc- 

 ess, or katabolism. The energy thus stored by the plant is released 

 to the animal for his use. The combination of these two forces is 

 one of the safest methods of distinguishing simple living matter 

 from non-living. 



From a physiologic standpoint we may group the attributes or 

 functions of living cells under ten headings: 



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

 toplasm. In the simplest cases it probably represents merelj^ 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. 



3. Digestion is the direct action of these secretions on the food 

 particles. It so changes the chemical composition of the food that 

 it becomes available for energy, or for the building up of the proto- 

 plasm. It is possible that these two uses are similar. 



4. Assimilation is the incorporation of the digested food par- 

 ticles into the protoplasm. In simple forms it is the equivalent 

 of anabolism. 



5. Katabolism is the process of liberating the energy stored up 

 in the protoplasm. It may be termed a tearing-down function, 

 in the sense that complex chemical compounds are torn down to 



