i 4 4 GROWTH 



It is not altogether easy to express, or in fact to determine, 

 these functional factors. The example of developing muscle 

 may help to explain. When a muscle is stimulated to contract, it 

 liberates energy in the form of electricity, heat, and mechanical 

 work. In the physiological act of contraction complex chemical 

 reactions occur with greater intensity than in the normal resting 

 muscle, and chemical by-products are formed in augmented 

 amounts. These steps change the physical constants of the mus- 

 cle protoplasm involved. There is an after-readjustment of the 

 water content of the muscle, of the percentage relations of the 

 nutritional elements in its environment, of the hydrogen and 

 other ions, of the oxidative processes, and in the end a variation 

 in the rate of new formation and growth of the very tissue that 

 has functioned. This is not the whole story in the living animal. 

 After the functional act of energy liberation, the chemical by- 

 products diffuse into other parts of the body, or are carried by 

 the circulation, and thereby become secondary factors in stimu- 

 lating or restraining activity and growth in such parts. These 

 effects are like an endless chain and are cyclic. In fact chemical 

 products of physiological activity constantly influence the de- 

 velopment and growth of all parts of the individual body. 



The physiological interrelationships within the body of the 

 human are also infinitely complicated and more or less cyclic. 

 The processes of activity occurring in certain parts or organs of 

 the body are constantly conditioned by what occurs in other 

 organs or parts. Such influences are of two great physiological 

 classes, (i) augmentative or stimulative, and (2) retarding or 

 inhibitive. These are illustrated by well-known physiological 

 processes such as the control of the heart rate, or the associated 

 inhibition and stimulation of antagonistic muscles, by the stimu- 

 lation and inhibition of the motility of the stomach, etc. We 

 know the details and the extent of inhibition and the external 

 expression of it in organs, but physiological science has yet con- 

 clusively to explain just how such restraint is accomplished. 



Analogies are to be had also from the plant world. Dr. 



