1. Where the membrane ruptures and the substances secreted are sent directly out into the digestive cavity. 2. Where the vesicles become free by strangulation, floating in the glandular or intestinal cavity, and ending by rupturing and coming into contact with the neighboring vesicles or with the food.
Absorbent cells.—Besides the glandular or secreting cells in Ptychoptera, there is between the two regions of the chyle-stomach lined with these cells a region about a centimetre long composed of absorbent cells. The absorbent cells are very large, polygonal, and contain a large nucleus, in which is a striated convoluted chromatic cord.
The food on entering the chyle-stomach is brought into contact with the products secreted in the proventriculus, in the first part of the chyle-stomach, and in the tubular glands. These products of secretion act on the food, extracting from them useful substances which they render soluble. These substances, after having been absorbed by the absorbent cells in the middle region of the stomach, undergo special modifications, and are transformed into solid products, which are situated at the bottom of these cells. Afterwards the alimentary substances freed from a portion of their useful substances are again placed in contact with the products of secretion in the distal part of the chylific ventricle, and reach the terminal part of the intestine.
“The products of secretion,” adds Gehuchten, “diverted into the intestinal canal do not come into immediate contact with the alimentary substances; they are separated from it by a continuous, structureless, quite thick membrane (the peritrophic membrane), which directly envelops the cylinder of food matters, extending from the orifice of the œsophageal valvule to the end of the intestine. Between this membrane and the free face of the epithelial lining there exists a circular space, into which are thrown and accumulate the excreted substances. The latter then cannot directly mingle with the aliments; but when they are liquid they undoubtedly pass through this membrane by osmose, and thus come into contact with the nutritive substances. It is the same with the products of absorption. The absorption of soluble products of the intestinal cavity is not then so simple a phenomenon as it was at first thought to be, since these products are nowhere brought into immediate contact with the absorbent cells” (pp. 90, 91).
The most recent authority, Cuénot, states that absorption of the products of digestion takes place entirely in the mid-intestine, and in its cæca when these are present. The mid-intestine exercises a selective action on the constituents of the food comparable to the action of the vertebrate liver.
LITERATURE ON THE PHYSIOLOGY OF DIGESTION
Davy, J. Note on the excrements of certain insects, and on the urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl, pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)
—— Some observations on the excrements of insects, in a letter addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854, pp. 18–32.)
Bouchardat, A. De la digestion chez le ver à soie. (Revue et Mag. de Zool., Sér. 2, 1851, iii, pp. 34–40.)
Lacaze-Duthiers, H., et A. Riche. Mémoire sur l’alimentation de quelques insects gallicoles et sur la production de la graisse. (Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)