NORMAL AND PATHOLOGICAL GROWTH 171 



For example, the growing layer of the outer skin or epi- 

 dermis may project deep down into the supporting tissues, 

 forming non-vascular masses very much larger than the usual 

 proportions. It is as though some overstimulative agency were 

 acting on the growth process in absence of adequate inhibiting 

 or restraining agency. Such overgrowth of epidermis or other 

 tissue in effect becomes parasitic on the rest of the body, de- 

 stroying the balance in development as between it and the nor- 

 mal tissues. This is in general the history of malignant or can- 

 cerous growths of whatever type and whatever origin. 



In spite of enormous financial expenditure and scientific 

 energy given to the solution of the cause of cancerous over- 

 growth, the causative facts are still unknown. In some tissues 

 cancerous growths seem to start as a result of prolonged and 

 repeated stimuli, either mechanical, or chemical, or combina- 

 tions of the two. Once free from the restraint of the inhibitive 

 processes, growth is rapid, voluminous, and destructive. 



Summarizing this chapter we may emphasize the following: 



1 . Growth as a physiological process is dependent upon some 

 deep, inherent characteristic of living protoplasm at present un- 

 known. It is aptly described as "the growth impulse." An abun- 

 dance of food and an adequate supply of vitamins provide the 

 essentials for growth. 



2. There are many organs, small in size but vital in impor- 

 tance, within the body of man and higher mammals, called endo- 

 crine glands, which secrete chemical hormones that profoundly 

 influence the rate and character of growth activity by a process 

 of augmentation or retardation. The average growth is there- 

 fore the algebraic sum of the stimulating and inhibiting proc- 

 esses going on within the body at all times during the life cycle. 



3. Variations in the growth curve, typical for various species 

 and man, depend largely on variations in the relative rate of 

 production offthe different stimulative and inhibitive hormones. 

 This is coincident with the fact that internal secreting glands 



