168 THE CAT. [CHAP. vi. 



by passing through the substance of limiting membranes, it is 

 obvious that this process must be synonymous with an increase of 

 the food's diffusibility, a quality acquired in part by a change in 

 its chemical composition, in part by its very minute subdivision. 



Minute subdivision is produced by mastication, by the contraction 

 of the walls of the alimentary canal, and by the influence of fluids 

 poured into that canal, and which reduce the fatty matter of the 

 food to the condition of an emulsion. 



Diffusibility is produced by a transformation of colloids into 

 crystalloids, starchy matters being changed into that highly soluble 

 crystalloid, sugar; and the albuminoid and gelatinoid substances, 

 being transformed into albumen-peptone and gelatin-peptone, both 

 of which are capable of ready absorption. These transformations 

 are effected by the agency of certain fluids which different parts 

 of the alimentary . organs secrete and pour into the alimentary 

 tract. 



These facts and considerations throw a certain light on the process 

 of alimentation. But any explanations to be thence derived are 

 manifestly most incomplete, because the very living membrane itself 

 can cause changes in the fluid itself as it passes through it, and the 

 living particles of parenchyma exercise a certain power of choice 

 with respect to the contents of the fluids in contact with them. 

 Such particles are not passive bodies, but active, living agents, and 

 their action no one has yet really explained. 



6. The processes of alimentation may then be SUMMARIZED as 

 follows : 



To support life by due repair of waste, and the maintenance of 

 the necessary body temperature, food is required of such a nature as 

 to furnish the substance of the tissues, and to serve as fuel. This 

 food must be minutely comminuted, or rendered soluble by me- 

 chanical action, and by the influence of suitable fluids. When this 

 process of digestion has been accomplished, the nourishing product 

 becomes more or less completely absorbed, and, passing into the 

 blood-stream, regenerates it, and through it supplies every part of 

 the frame with fresh material, which is taken up by internal 

 assimilation or intussusception, and transformed into the substance 

 of the living body the non-nutritious, non- absorbed residua being 

 discharged. 



This great function is subserved by an elaborate apparatus, 

 commonly known as the stomach, intestine, &c., with their annexed 

 organs. It may be shortly described as a convoluted tube of 

 different capacity in different parts, passing from one end of the 

 body to the other, with two terminal apertures and with muscular 

 walls, the fibres of which are so arranged as, by their regular, 

 alternate contraction and relaxation, to drive the contents of the 

 tube onwards from its anterior to its posterior termination. 



The anterior part of the tube is enlarged and specially modified 

 to serve for the reception of the food, its subdivision and preparatory 

 moistening by certain fluids. This is the luccal cavity, or mouth, 



