ABSORPTION. 599 



animals, whether vertebrate or non-vertebrate, such as the voracious Fishes, 

 and the carnivorous Mollusca and Coelenterata, there is little or no necessity 

 for a salivary fluid capable of transmuting starch ; whilst even in the lowest 

 animals, fatty and albuminoid substances are essential constituents alike of 

 the bodies and of the food. The low temperature of these animals, and the 

 higher temperature of the starch-feeders generally, are interesting facts, in 

 connection with the heat-producing power of amylaceous diet. 



In the unicellular Protozoa, whether, as in the Infusoria, there exist a short 

 tube leading into their interior, or as in the Sponges and Rhizopods, no di- 

 gestive cavity at all, solid food must also be dissolved by some action of the 

 animal, before it is absorbed ; though these universally aquatic creatures may 

 be partly nourished by materials already dissolved in the surrounding me- 

 dium. 



In the case of the parasitic Gregarinida, and even of certain of the annuloid 

 Entozoa found in the alimentary canal of other animals, the nutrient sub- 

 stances absorbed are probably those which have already been digested, and so 

 prepared for absorption by the gastric and other secretions of those animals, 

 a sort of vicarious digestion being here employed. In those Entozoa, how- 

 ever, which infest other organs or tissues, such as the air-tubes, muscles, 

 brain, and interior of the eyeball or bloodvessels, probably little or no digestive 

 change of the nutrient materials is required ; the nutritive function consists 

 merely of imbibition and assimilation, observed in the ultimate nutritive pro- 

 cesses in the higher animals, and digestion is merged in nutritive absorption. 



ABSORPTION. 



By the process of digestion, the food is reduced to a compound ali- 

 mentary basis, composed of aqueous, saline, extractive, mucilaginous, 

 saccharine, amylaceous, oleaginous, and albuminous matters, some- 

 times mixed with alcoholic, ethereal, acid, pungent, odoriferous, and 

 coloring substances. The materials of this complex pabulum, whilst 

 retained within the digestive cavity, remain, strictly speaking, external 

 to the living frame ; but a process immediately ensues, by which they 

 are, sooner or later, taken up into, and enter the living tissues ; this 

 is termed Absorption. The chief object of this process of the absorp- 

 tion of food, is the introduction of new material, for the repair of the 

 continuous waste of the living body. 



Absorption, however, considered as a physiological function, con- 

 sists of more than the mere taking up of nutrient materials from the 

 interior of an alimentary canal, or of a simple digestive cavity, or at 

 the surface, of a unicellular animal organism. It includes that general 

 process by which all external soluble substances, whether solid, fluid, 

 or gaseous, beneficial or poisonous, nutrient, stimulant, or respiratory, 

 are introduced into the tissues of the body, through any natural or 

 artificial surface whatever. Moreover, it comprehends, in part at 

 least, another process, by means of which portions of the living tissues 

 are themselves removed, or absorbed, within the body. The former 

 of these two processes is sometimes named general absorption, and the 

 latter, intrinsic or interstitial absorption. Intrinsic absorption is es- 

 sentially a nutritive process. The term extrinsic may be applied both 

 to general absorption and to the absorption of food. 



