The Lumleian Lectures on the Digestive Ferments, and the Preparation and Use of Artificially Digested Food
Authors: W. Roberts
Journal: BMJ (1880)

LECTURE I.DIGESTION has been usually regarded as the special attribute of animals.They receive into their alimentary canal the food which they require for their sustenance in a crude form.It is there subjected to the action of certain ferments which transmute its elements, by a peculiar chemical process, into new forms which are fitted for absorption.Looked at in this restrictive sense, plants have no digestive function.They possess no alimentary canal, nor any vestige of a digestive apparatus.But when the matter is examined more profoundly, it is seen that plants digest as well as animals, and that the process in both kingdoms of nature is fundamentally the same.In order to understand this generalisation-which wuas first propounded by Claude Bernard, and constitutes one of the most important fruits of his splendid labours*-it is necessary to recognise digestion under two types or conditions, namely, a digestion which takes place exteriorly at the surface of the organism, and a digestion which takes place intersti- tially in the interior of the organs and tissues.Exterior digestion is that common process with which we are fami- liar as taking place in the alimentary canal of animals, by which the crude food introduced from without is prepared for absorption.Interstitial digestion, on the other hand, is that more recondite process by which the reserves of food lodged in the interior of plants and animals are modified and made available for the purposes of nutrition.These two types of digestion are essentially alike, both as regards the agents and the processes by which they are carried out; and, although one type of digestion is more developed in the animal kingdom, and the other type more developed in the vegetable kingdom, both types are represented in the two kingdoms, and bear witness to the fundamental unity of the nutritive operations in plants and animals.I shall only be able to indicate in outline the facts and arguments on which Bernard sought to establish these propositions.ExteriorDigestion.-W_"e all know that the alimentary canal is simply a prolongation of the external surface; that the skin is continued, at each extremity, without a break, into the alimentary mucous membrane.Accordingly, the processes which take place in the digestive tube are, strictly speaking, as much outside the body as if they took place on the surface of the skin.Upon this inner surface, if I may so call it, are poured out the digestive juices, charged with the ferments which are the special agents of the digestive processes.This is the common condition of exterior digestion as it occurs in animals; but it is not the only con- -dition.Among some of the lowest members of the animal series, a per- manent alimentary canal does not exist.In the amceba, any portion of the exterior is adapted for the reception of food.The morsel sinks into a depression formed on the surface at the point of impact; it is digested in this improvised stomach; and the indigestible portions are expelledl through an improvised anus.Among plants, exterior digestion is a nmuch less prominent feature than among animals; but examples of its occurrence and evidence of its importance are not difficult to point out.In the lowest orders of plants-fungi and saprophytes, which are devoid of chlorophyll-extenor digestion is probably a function of prime necessity.In all likeli- hood, their carbon-containing food is only absorbed after undergoing a process of true digestion.The transformation of cane-sugar by the yeast-plant is a striking example-though a distorted one-of exterior digestion.Cane-sugaris acrudeformof food both toplantsand toanimals, and requires to be transformed into invert-sugar (a mixture of equal parts of dextrose or grape-sugar and laxvulose or fruit-sugar) before it Claude Bernard, Lf ons sur tes J'/ubaomd,ses de MaVie, tome ii; edited after his death by Dastrc.Paris: 1879.539 can be made available for nutrition.The yeast-plant is no exception to this rule; and, when )laced in a solution of cane-sugar, it is under the necessity of transforming that compound into invert-sugar before it can use it for its profit in fermentation.This transformation is effected by a soluble ferment attached to the yeast-cell, which can be dissolved from it by water.We shall see later on that a similar ferment exists, for a similar purpose, in the small intestine of animals, having the same property of changing cane-sugar into invert-sugar.Even amiong the higher plants, exterior digestion is not quite unknown.