RIGIDITY OF TISSUES 87 



The hydrophilic properties of sodium and calcium soaps have 

 already been mentioned. Their behaviour in emulsion-making 

 throws light on some peculiar problems in physiology. Loeb and 

 his co-workers found that certain marine organisms died when 

 put into fresh water. This will not appear surprising to the 

 student who remembers the phenomena of endosmosis, e.g. plasmo- 

 lysis, haemolysis, etc. That this explanation is not correct is 

 shown by putting the organisms into solutions of sodium chloride 

 or of calcium chloride having the same osmotic pressure as sea 

 water. If, however, the organisms which would have been 

 killed by immersion in these isotonic solutions were placed in a 

 solution having a definite ratio between the amount of sodium 

 and calcium present, life was maintained quite normally. All 

 protoplasm may be considered as an emulsion of lipoid material 

 in a colloidal-crystalloidal complex. The presence of the sodium 

 soap formed by interaction with the lipoids causes the formation 

 of a lipoid-in-water emulsion while the calcium soaps emulsify 

 water-in-lipoid. The two types of emulsion thus formed are in 

 equilibrium with an environment containing a definite Na/Ca 

 ratio, that of sea water. Alteration in this ratio upsets the balance 

 between the two types of emulsion and causes the cessation of 

 growth and subsequently of life (see Nerve, Chap. XVII.). 



The rigidity of tissue is to a large extent due to their emulsion 

 character. We have up till now considered protoplasm as a 

 liquid, arguing that it is so because it shows the phenomena of 

 surface tension, because it allows the ready diffusion of crystal- 

 loids into and through it, and because it reacts chemically as a 

 liquid. On the other hand, tissues, as we handle them, are more 

 or less rigid, having elasticity and definiteness of form. Do 

 Pickering's solid emulsions and the Na/Ca ratio not suggest a 

 fairly plausible explanation of this double nature of protoplasm ? 

 The " softening " of tissues observed in various pathological 

 states may be due to the breaking of the protoplasm-emulsion 

 from any cause (Part II.). 



Our food materials as well as our tissues are colloidal complexes. 

 They are derived in part from the animal, in part from the vege- 

 table kingdoms. 

 A. Animal foods may be classified as : 



(1) Milk and its products cream, butter, and cheese. 



(2) Flesh. 



(3) Eggs. 



(1) Milk is a fine emulsion of fat in a protein-colloidal solution. 



