RIGIDITY OF TISSUES 111 



pattern I'roni the parallel planes in the niicrocrystals, lia\ ing une 

 direction in conniion, will appear as concentric rinj^s. The 

 distance between the reflecting planes is twice the length of the 

 nioleeides composing the crystals, so that if the length of the fatty 

 acid chain in a soap is increased, the parallel reflecting planes in 

 the crystals produced when the myelin form contracts will div^crge. 

 The amoimt of this divergence is just over 0-1 /x/x for every addi- 

 tional earl)on atom added. 



When the soap contracts, that is, when the liquid crystals within 

 the myelin sheath are converted into true crystals and so become 

 more closely packed together, heat is evolved. This heat is, of 

 course, absorbed during the converse process. The former is an 

 example of an exothermic, the latter of an endothermie reaction. 



As mentioned above, under the influence of the alkali set free 

 by the ionisation of the soap in contact with water, imbibition is 

 induced. Water passes in through the myelin membrane and is 

 incorporated in the soap, just as water is imbibed by gelatin and 

 laminar ia. This water-in-soap colloid is neutral or faintly acid to 

 phenolphthalein in contrast to a soap-in-water sol, which is 

 intensely alkaline. 



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

 character. WV 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 (^f 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 ? 

 A cell is a water-in-protein complex, while its secretion (of similar 

 composition) is of the protein-in-water type. 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 deri\ed 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. 

 («) The fat globules each seem to be enveloped by a covering 

 of adsorbed protein. 



(6) The chief protein in milk is easeinogen, a phospho-proteiu 



