Some observers have attempted to verify the action of this delicate muscular apparatus dur ing life. Gruby and Delafond first instituted such observations on a variety of domestic animals ; and they have since been repeated by Bruecke and Koelliker. All of these authorities agree in remarking an alternate shortening and elongation of the villus: —a change of form which is so rapid and marked, that there need be little hesitation in attri buting it to a corresponding contraction and relaxation of these unstriped fibres. But the phenomena seen in the course of such vivisections cannot be safely accepted as those of the healthy animal in its natural state.* Such a caution is still more applicable to those contractions hy which the villi share in that irregular movement of the intestine already described as a kind of rigor mortis. On exposing the villi of an animal soon after death, they gradually become shorter and wrinkles or folds. A more minute examination shows that these folds consist of epithelium, which has separated from the basetnent mem brane at the points that correspond to the greatest projection between the contiguous wrinkles Cfig,. 264,, 2.) A closer adhesion of those columnar cells which occupy the free extremity of the villus, frequently causes this part to be defined, as a shallow funnel, by the neighbouring separated cells. While in other instances, a variable length of the whole villus withdraws from its cellular investment with such uniformity, as to leave the extremity of the latter empty, smooth, and uninjured (fig. 264,, I.), like the finger of a glove. Frequently, how ever, some of the epithelial cells are detached.
It is obvious that all these appearances are referrible to a contraction of the un striped fibres within the villus, withdrawing the substance of the latter from its epithelial in vestment. The movements which often ac company these changes resemble those above mentioned as beheld during life ; and consist of shortening or elongation, to which are some times added lateral displacements. The date of their occurrence is limited to the period which immediately succeeds death. And their duration is rarely protracted beyond a few minutes.
Changes in the villi during digestion. —Dur ing the act of digestion, the villi undergo cer tain noticeable alterations. At this period, they receive an increased afilux of blood ; and beconie both larger and softer. They. ac quire a greater opacity ; so as to appear whiter by reflected, and darker by transmitted, light. The nuclei and cells which occupy their in terior are greatly increased in number and dis tinctness. And, finally, after the ingestion of food containing the usual fatty ingredients, a portion of these may be found occupying the interior of the villi themselves.
The process by which fatty matter pene trates the villus to enter the lacteal in its centre, deserves our special attention, from the fact that it constitutes the origin of the chyle. At present we shall limit ourselves
to a description of the appearances actually observed, in connection with the mucous membrane of the alimentary canal.
The first step towards the absorption of the fatty matter, consists in its entry into the epithelium which invests the exterior of the villus. Each columnar cell of this covering is gradually filled by a large oil globule; which occupies the whole of its cavity-, with the ex ception of that small portion devoted to its nucleus. This change first implicates a few scattered epithelia ; and, by rendering them more refractile, often causes various parts of the surface of the villus to offer a curious trast of bright spots (b,fig. 263.) and darker a A. a-4 kJ J. ill L. *at./ 1.1 intervals.* Gradually, however, all the cells become similarly affected; so that the entire villus assumes the altered appearance just alluded to.
The next step towards the absorption of the fatty matter consists in the minute subdivi sion of the single oil globule (c, fig. 266.) which occupies the epithelial cell. The way in which this process occurs is unknown : but the result of the change is to give the columnar cell a darkly granular appearance (a, fig 266.), in which we may often distin guish separate, though minute, fatty mole cules. These molecules are next found in the substance of the villus itself, though chiefly tosvards its surface and free extre mity ; — to the apex of which latter part they are often limited.f From the sub stance or matrix of the villus, the molecules of fat are then transferred to the lacteal trunk occupying its centre ; which, in the most fa vourable instances, they define as a slender column of dark fatty granules.
How far the above process constitutes a mere act of physical imbibition, it is difficult at present to determine. But that it is so, at least in part, can scarcely be doubted. For the experiments of Matteucci (which are con firmed in all their essential particulars by Valentin §) prove that, when a dilute alkaline solution and a faintly alkaline fatty emulsion are separated from each other by an animal membrane, ditrusion occurs between them. And the circumstances actually present in the intestine are even more favourable to such a transit than those which obtained in the experiments of these observers. The lyniph and blood are sometimes more alkaline than the solution which they made use of. The degree in which the tenuity of the delicate cell-wail of the villas exceeds that of the compound membrane forming the diffusive septum in their experiments, would propor tionately favour the resulting transit of the separated fluids. And since the continuous movement of the chyle is probably aided by forces independent of any mere act of dif fusion, the force of suction thus added must itself conditionate a more active transit than that which they witnessed in the inert en dosmometer.