CHAP. VL] THE CATS ALIMENTARY SYSTEM. 1G7 



the product being discharged from some surface external or 

 internal. 



But in fact it is not the blood alone which is in all cases the direct 

 source of nutrition, since the blood has the power of replenishing 

 itself and repairing its losses out of the fluids obtained from the 

 food. The intimate way in Avhich assimilation takes place, is 

 named INTUSSUSCEPTION, to distinguish it from any growth which may 

 take place by mere external addition as when a crystal grows, 

 while suspended in a suitable medium, by the deposition of fresh 

 matter on its surface. 



Another process, which is ancillary to nutrition and secretion, is 

 termed ABSORPTION, which is the generic term applied to the intro- 

 duction into any tissue of the body, of substances external to it, 

 and thus nutrition, or assimilation, itself is, in fact, one form of 

 absorption. The process of absorption is aided by the physical 

 properties termed cndosmosis and cxosmoxis, terms which denote the 

 passage of fluids in opposite directions through dead animal 

 membranes ; different fluids, when thus divided, tending to pass 

 through to the other side of such membranes with different degrees 

 of rapidity. 



Dialysis is the term used to denote this movement of transfusion, 

 irrespective of its direction, and therefore includes both endosmosis 

 and exosmosis. 



4. It has been found that different substances may be arranged 

 in two classes according to their diffusibility, and that this division 

 coincides with certain other characters which the two classes, termed 

 respectively CRYSTALLOIDS and COLLOIDS, present. All crystalloid 

 bodies are crystallisable ones. When dry, they are hard, rigid, and 

 quickly soluble ; their solutions are never viscous, they are always 

 more or less sapid, and they are highly diffusible. Colloids do not 

 crystallize, and when dry they are tough. They dissolve slowly, and 

 their solutions are more or less viscous ; they are insipid, and they 

 diffuse with difficulty. Albuminoid and gelatinoid substances are 

 colloids. 



Dialysis doubtless takes place in the living body : as in secretion, 

 nutrition, and absorption, and it is possible that some such process 

 may be the cause antecedent to muscular contraction. All salts and 

 other crystalloid matters, whether useful, indifferent, or hurtful, 

 readily find their way into the substance of the body from the 

 alimentary canal, but, as we shall see later, this ready penetration 

 of very diffusible substances is not the same thing as true intestinal 

 absorption where a selective power is manifested. This latter active 

 kind of absorption is, as has been already said, analogous to 

 secretion. 



5. The consideration of the distinctions which exist between 

 colloids and crystalloids leads ns to the last preliminary considera- 

 tion, namely, to that of the process of DIGESTION. This process 

 consists in the reduction of food to a state in which it can be readily 

 taken up into the system, and since it cannot be so taken up except 



