CHARACTERISTICS OF THE ANIMAL BODY 25 



yield their quota of energy. Complementary to this is the excre- 

 tion of the waste-products of energy production 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 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 Uving 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 a good example of this attribute. 



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

 extreme economic value. It needs little discussion, as all can wit- 

 ness it, and further, because the scientist is but httle nearer its 

 fundamentals than the layman. 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 com- 

 plete cessation of the vital processes. This is spoken of as senile 

 degeneration. For each species there is a limit to this period of 

 growth: most small animals develop completely in a year, horses 

 take four to five years, and man approximately twenty-one years. 

 The average height of man is 5 feet, 8 inches. The average 

 weight of the horse is between 1000 and 1100 pounds. Giants and 

 dwarfs are always abnormal productions and represent pathologic 

 changes. These abnormalities are not often transmitted to the 

 offspring. 



9. Reproduction is probably the most remarkable and interest- 

 ing of all the attributes of the living animal. Detailed considera- 

 tion of this characteristic will be left until the reproductive organs 

 of the body have been studied, when it can be more easily under- 

 stood. 



10. Response to environment is the power that induces the pro- 

 duction of substances that help neutralize disease, repair wounds, 

 and adapts the body to its physical surroundings. 



