220 NUTRITION 



atoms or ions or lesser molecules (whichever it may be) within itself so 

 that growth is possible. Conjectures as to the exact mode of this proteid- 

 growth are well-nigh vain, so little do we know of the structure of the 

 biomolecule or vital group, while of the proteid molecule itself we know 

 but little more. One conjecture, however, seems reasonable namely, 

 that this protoplasmic unit or group (whatever in exact physical terms 

 it may prove to be) increases in complexity by development and accretion 

 up to a certain limit and then breaks up into daughter-units (each based 

 on a cyanogen-root), which thereupon grow and split in turn. The 

 developments which occur in this (protoplasmic) tissue-unit take place 

 probably all through it and not on its periphery only. In other words, 

 growth is real, inherent development and not mere accretion from without, 

 for else the complexities of metabolism could not be accounted for 

 (Hering). By some method the food that serves as the means of ana- 

 bolism supplies particles of just the required composition, etc., to the 

 tissue-units, and these thereupon grow and split up and so form a new 

 particle of protoplasm. Finally, unknown multitudes of these biogenic 

 particles become a tissue-cell, the morphological unit, with its organiza- 

 tion of nucleoplasm and cytoplasm, familiar in every perfect cell. 



It cannot be doubted that water, inorganic salts, fats, and carbohy- 

 drates take part simultaneously with proteid in the anabolic tissue- 

 growth. It seems probable, however, that the fats and carbohydrates 

 are not so intimately concerned in the formation of new protoplasmic 

 units as are the proteid, the inorganic salts, and water. The proteid 

 doubtless forms the core or basis of the unit, and either contains or 

 carries the salts. The water is always inherent and basal in protoplasm. 

 The fats and carbohydrates are perhaps related to this cyanogenic unit 

 as necessary foodstuffs to lend it strength, thereby making its manifold 

 metabolism and activity possible. Each sort of tissue-unit has the 

 power of taking on from the lymph precisely those molecules of food 

 which it needs, and each has the means of preventing the attachment 

 of those it does not require. In this way it is self-perpetuating. At 

 the same time, still more marvellously perhaps, every vital unit has 

 the faculty of developing to meet those entirely novel requirements 

 which new habits, new uses, and new environments are continually 

 making essential in animal life. How else can we account for the 

 molecular development of a scholar's brain, the cunning of the muscles 

 of a human hand, or the adaptation to new needs of the organ of Corti 

 in a musician's ear? 



The remainder of this chapter deals with manifestations of metabolism 

 more largely in its katabolic phase. 



Secretion. Between the process we have considered as growth and 

 the other only less general phenomenon we shall now describe as 

 secretion the differences are mainly arbitrary, but obviously with this 

 exception: While in tissue-growth the new-formed substance becomes 

 and remains for a time a part of the mother-tissue, in secretion the 

 product must of necessity be removed with more or less promptness. In 



