CHAP. VI.] THE CAT'S ALIMENTARY SYSTEM. 1G7 



— the product being discharged from some surface external or 

 internal. 



13ut in fact it is not the blood alone Avliicli is in all cases the direct 

 source of nutrition, since tlio blood has the power of i-cplenishing 

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

 food. The intinuite wa}^ in which assimilation takes place, is 

 named iNTussrscErxiox, to distinguish it from any growth which may 

 take place by mere external addition — as Avhon a crystal grows, 

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

 matter on its surface. 



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

 termed ABSonrxiON, 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 cn(/o>i/)w.sis and ctv-^inosis, terms which denote the 

 passage of fluids in opposite directions through dead animal 

 membranes ; diff'erent fluids, Avhen thus divided, tending to pass 

 through to the other side of such membranes "with dLfterent degrees 

 of rapidit)''. 



Dia///sis 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 ciystallisable ones. When dry, they are Imrd, rigid, and 

 quickly soluble ; their solutions are never viscous, they arc 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, xilbuminoid and gelatinoid substances are 

 colloids. 



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

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

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

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

 readily And 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 us to the last preliminary considera- 

 tion, namely, to that of the process of digestiox. 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 



