DISTURBANCES OF GROWTH 241 



from the time the ovum is fertilized. Continuity or discontinuity of 

 growth at different periods in its history, the manner and extent to which 

 the tissues differentiate and specialize with respect to function, as well 

 as the ultimate size which the organism will attain, are determined by 

 peculiar differences in the structure of the protoplasm of different species. 



There is an optimum environment for the cells which compose the tis- 

 sues, in which they will function most satisfactorily, and for the maximum 

 time without developing faults in their mechanism, which we recognize as 

 metabolic disturbances, by means 01 clinically recognizable alterations of 

 function. Abnormal chemical reactions within the body lead to the ex- 

 cretion of products of perverted metabolism. 'Histological changes in the 

 tissues, which may or may not become manifest later by gross anatomical 

 changes, occur in certain conditions . as scurvy, beri-beri, rickets and 

 xerophthalmia. This environment, which is furnished by the blood, 

 lymph, spinal fluid and other fluids of localized occurrence, with which the 

 tissues are in contact, approximate most closely the ideal when the food 

 supply is optimum in its composition. 



There are safeguards to maintain the blood of satisfactory composition. 

 The most efficient among these is the mechanism which preserves the neu- 

 trality of the tissues and body fluids. If the food is not of the most satis- 

 factory kind the intestinal mucosa may exert to some extent a selective 

 action in absorbing nutritive or other substances. The liver is very ef- 

 fective in transforming certain useless or toxic substances into harmless 

 or less harmful bodies. The kidneys and the intestinal mucosa promptly 

 excrete many substances whose presence in the environment of the living 

 tissues are objectionable. These agencies are, however, of very limited 

 efficiency and fail to maintain in all circumstances an optimum composi- 

 tion of the circulating and nutrient fluids of the body. It is not suffi- 

 cient, therefore, that all the essential nutritive complexes and elements be 

 present in the food. These should be introduced into the alimentary 

 tract in the right proportions to meet the needs of the tissues. Either 

 shortage or excess of one or another substance which is indispensable 

 for the upkeep of the body may do harm. Even slight departures from the 

 optimum in the character of the food supply, when maintained over con- 

 siderable periods, leave their mark on the physiological well-being of the 

 individual in a manner which can be easily recognized by appropriate 

 methods of experiment. This fact has not been sufficiently recognized by 

 clinicians or physiologists, and accordingly it is of special importance that 

 we should illustrate the tendencies of faulty nutrition of a mild type, when 

 persisted in, to cause disturbances of metabolism and perversion of func- 

 tion. 



When any of the conditions essential for the normal development 

 of an organism are sufficiently altered, growth is interfered with. The 

 earliest interference with growth appreciated by physiologists was doubt- 



