ib. o, with the spleen. In the Kangaroo it is of moderate size, continued loosely from the stomach to the transverse colon, but not extended beyond that part. The posterior layer lies between the stomach and the intestines, and exemplifies one of the uses of the epiploön, as it prevents these parts from interfering with each other's motions. The anterior layer generally contains more or less fat. In the Petaurus the epiploön is continued from the great curvature of the stomach and the commencement of the duodenum. In the Phalangers it is of considerable extent and is usually loaded with fat. In the Opossums I have found it generally devoid of fat, when this substance has been accumulated in other parts. In the Phascogales and Dasyures the epiploön is of moderate size, and contains little or no fat. The epiploön is attached to the lower arches of the several divisions of the stomach in Cetacea, is always devoid of fat, and is of limited extent: the subdivided spleens, fig. 355, h, i, are scattered in it, as in a net: it is in parts reticulate. The epiploön is small and does not cover the intestines in Sirenia, Proboscidia, and Perissodactyla. It is, also, of limited extent in the Hog-tribe. In fatted Sheep it is larger and is reticulated with adipose matter. It is attached, in Ruminants, to the right side of the left division of the rumen, and along its anterior or ventral convexity, passing from the right of this to the abomasus and the beginning of the duodenum: it does not cover the intestines, and is commonly found crumpled up beneath the paunch. The reticulate structure of the great omentum appears to be natural and pretty constant in the Dog and some other Carnivora: in the Seal the omental fold is thin and devoid of fat.

The peritoneum lining the elastic ventral wall of the abdomen in the Elephant and Rhinoceros is of unusual thickness and strength, the areolar tissue connecting it to adjacent structures presents an aponeurotic firmness: the free surface of the serous membrane I found to be white and opaque: it is generally transparent and opaline or colourless. In some hibernating Rodents a fold of peritoneum extends forward from each lumbar region, covering the lateral convolutions of the intestine as far as the umbilicus, and towards the beginning of winter becoming the seat of an abdominal deposit of fat: they may serve with the ordinary omentum the double purpose of nonconductors of heat and a store of nutriment. [Since Sheets T-DD were printed off, the excellent Paper CLXXXVI" has appeared, showing that the deciduous teeth of the mole, though too minute to seem of use, are not shed until after birth. In other respects Mr. Spence Bate confirms the talpine formula given at p. 304.]

1 v. p. 37.

CHAPTER XXXI.

ABSORBENT SYSTEM OF MAMMALIA.

§ 341. Lacteals.-In Mammalia the intestinal villi constitute a modification of surface intimately related to the formation and more especially to the absorption, of chyle. Such villi, e. g. of a calf killed after being fed with milk, exhibit, when magnified

390

as in fig. 390, a central canal, dilating towards its end, c, white or opaque with chyle: it appears to be an excavation in the substance of the villus, and the only definite tunic is the limitary membrane, a; from which the epithelium (shown in fig. 350, o) has been removed. The columnar cells of which this epithelium is composed are the direct agents of absorption. Each cell becomes gradually filled by a clear globule of refractive fluid, like oil. The scattered cells which are first filled, cause parts of the surface of the villus to glisten, as in fig. 391, in contrast with the darker tracts of unfilled cells. The oil-like globule next undergoes changes, represented in the cell-series, fig. 392, which mainly consist in a subdivision or reduction of the globule, d, to the granular state in a, the nucleus of the columnar cell remaining unchanged. These granules, or molecules, escape by rupture or solution of the cell-wall, penetrate the limitary membrane, become aggregated in the basal tissue of the villus, and finally enter the lacteal canal. Dead animal membrane does

Intestinal villi with lacteal canal, Calf, magn. CLXXVIII".

391

not prevent the effects of the ever-present, ever-active force which manifests itself, e.g., in the combination of an alkaline solution with a less alkaline fatty emulsion previously separated by such membrane and the cell-wall would offer much less physical resistance to the diffusive interchange than the membrane used, e. g., in Matteucci's experiments. But, besides the act of physical imbibition, with which the intussusception of aliment by monads or nucleate cells is closely related if not identical, there are also assimilative changes effected by these organites. Viewed by the microscopic aids of the last century they were thought to be orifices by which the chyle was sucked up and then conveyed by beginnings of the lacteal absorbents to the central space or trunk,'

of which Cruikshank saw but one in Intestinal villus, Dog, magn. 400 diam each villus' of a female who had died.

These

392

d

suddenly a few hours after a full meal (CLXXVIII"): occasionally two have been seen with looped unions in one villus: in Mammals with broader villi the chyle-cavity is reticulate. trunks are, however, the first definite absorbent channel, and, acquiring proper walls, unite together at the roots of the villi to form a network at the areolar basis of the mucous membrane, whence branches proceed to perforate the

muscular coat, and take a trans- Epithelial cells of a villus, during absorption verse course to the line of attach

of fat, magu. 350 diam. CXLVIII".

ment of the mesenteric layers. There are, also, superficial absorbents of the serous coat, which affect a longitudinal course and unite with the lacteals in their passage to the areolar interval of the layers of the mesentery: here they traverse the mesenteric glands, and progressively unite into a plexus surrounding the superior mesenteric artery. The lacteals and lymphatics from the cæcum and colon, which also traverse absorbent ganglions or

1 CLXXVII". p. 104.

CHAPTER XXXI.

ABSORBENT SYSTEM OF MAMMALIA.

§ 341. Lacteals.-In Mammalia the intestinal villi constitute a modification of surface intimately related to the formation and more especially to the absorption, of chyle. Such villi, e. g. of a calf killed after being fed with milk, exhibit, when magnified

390

as in fig. 390, a central canal, dilating towards its end, c, white or opaque with chyle: it appears to be an excavation in the substance of the villus, and the only definite tunic is the limitary membrane, a; from which the epithelium (shown in fig. 350, o) has been removed. The columnar cells of which this epithelium is composed are the direct agents of absorption. Each cell becomes gradually filled by a clear globule of refractive fluid, like oil. The scattered cells which are first filled, cause parts of the surface of the villus to glisten, as in fig. 391, in contrast with the darker tracts of unfilled cells. The oil-like globule next undergoes changes, represented in the cell-series, fig. 392, which mainly consist in a subdivision or reduction of the globule, d, to the granular state in a, the nucleus of the columnar cell remaining unchanged. These granules, or molecules, escape by rupture or solution of the cell-wall, penetrate the limitary membrane, become aggregated in the basal tissue of the villus, and finally enter the lacteal canal. Dead animal membrane does

Intestinal villi with lacteal canal, Calf, magn. CLXXVIII".

391

not prevent the effects of the ever-present, ever-active force which manifests itself, e. g., in the combination of an alkaline solution. with a less alkaline fatty emulsion previously separated by such membrane and the cell-wall would offer much less physical resistance to the diffusive interchange than the membrane used, e. g., in Matteucci's experiments. But, besides the act of physical imbibition, with which the intussusception of aliment by monads or nucleate cells is closely related if not identical, there are also assimilative changes effected by these organites. Viewed by the microscopic aids of the last century they were thought to be orifices by which the chyle was sucked up and then conveyed by beginnings of the lacteal absorbents to the central space or trunk,'

of which Cruikshank saw but one in Intestinal villus, Dog, magn. 400 diam each villus' of a female who had died

392

These

suddenly a few hours after a full meal (CLXXVIII"): occasionally two have been seen with looped unions in one villus: in Mammals with broader villi the chyle-cavity is reticulate. trunks are, however, the first definite absorbent channel, and, acquiring proper walls, unite together at the roots of the villi to form a network at the areolar basis of the mucous membrane, whence branches proceed to perforate the

muscular coat, and take a trans- Epithelial cells of a villus, during absorption verse course to the line of attach

of fat, magu. 330 diam. CXLVIII".

ment of the mesenteric layers. There are, also, superficial absorbents of the serous coat, which affect a longitudinal course and unite with the lacteals in their passage to the areolar interval of the layers of the mesentery: here they traverse the mesenteric glands, and progressively unite into a plexus surrounding the superior mesenteric artery. The lacteals and lymphatics from the cæcum and colon, which also traverse absorbent ganglions or

1 CLXXVII". p. 104.