26 PRINCIPLES OF VETERINARY SCIENCE 



lar enzymes, some of which are extracellular, while others are 

 intracellular. 



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

 particles. Only food that is digestible by its enzymes can be 

 used by the cell. It so changes the chemical composition of the 

 food that it becomes available for energy, or for the building up 

 of the protoplasm. 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 

 yield their quota of energy. Complementary to this is the 

 elimination or excretion of the waste-products of energy produc- 

 tion and also those of digestion and assimilation. 



6. Irritability is the sensitiveness that the protoplasm shows 

 to stimulation by outside objects. It is the foundation of the 

 forces that have become so highly developed and specialized in 

 the muscle- and nerve-cells. This response to a stimulus is more 

 highly developed in the mammalia than in reptiles, and in the 

 latter than in fishes. 



7. Motility, or the power of movement, is one of the chief means 

 of recognizing living matter. Its simplest manifestation, a 

 flowing of the protoplasm due to changes in surface tension, is 

 represented in the white cells of the blood that act as scavengers 

 and destroy disease-producing organisms. Contractility, a 

 function of muscle-cells, is an example of modified motility. 



8. Growth is one of the essentials that has come to have an 

 extreme economic value. It needs little discussion because all 

 can witness it, and further, as the scientist is but little nearer its 

 fundamentals than the layman. It has been said to represent 

 the excess of anabolism over katabolism. Living things grow by 

 absorbing new material from without and transforming it into 

 substances of their own bodies. Growth takes place only through 

 a certain period, being rapid during youth, about stationary at 

 maturity, and negative during old age. In fact, there is usually 

 a gradual decay and wasting away during old age, finally leading 

 to "a complete cessation of the vital processes. This is spoken 

 of as senile degeneration. For each species there is a limit to 



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