of what had been removed, and the denuded spot had contracted to much less than its original dimensions."

Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stomach, is sufficient to neutralize the acidity of the gastric juice; and as may be gathered from what has been previously said, the neutralization of the acidity of the gastric secre

ig. 189. Auerbach's nerve-plexus in small intestine. The plexus consists of fibrillated substance, and is made up of trabecule of various thicknesses. Nucleus-like elements and ganglion-cells are imbedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Klein.)

tion is quite sufficient to destroy its digestive powers; but the experiments adduced in favour of this theory are open to many objections, and afford only a negative support to the conclusions they are intended to prove. Again, the pancreatic secretion acts best on proteids in an alkaline medium; but it has no digestive action on the living intestine. It must be confessed that no entirely satisfactory theory has been yet stated.

VOMITING.

The expulsion of the contents of the stomach in vomiting, like that of mucus or other matter from the lungs in coughing, is preceded by an inspiration; the glottis is then closed, and immediately afterwards the abdominal muscles strongly act; but here occurs the difference in the two actions. Instead of the vocal cords yielding to the action of the abdominal muscles, they remain tightly closed. Thus the diaphragm being unable to go up, forms an unyielding surface against which the stomach can be

pressed. In this way, as well as by its own contraction, the diaphragm is fixed, to use a technical phrase. At the same time the cardiac sphincter-muscle being relaxed, and the orifice which it naturally guards being actively dilated, while the pylorus is closed, and the stomach itself also contracting, the action of the abdominal muscles, by these means assisted, expels the contents of the organ through the œsophagus, pharynx, and mouth. The reversed peristaltic action of the œsophagus probably increases the effect.

It has been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is effected solely by the pressure exerted upon it when the capacity of the abdomen is diminished by the contraction of the diaphragm, and subsequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdominal muscles alone is sufficient to expel matters from an unresisting bag through the œsophagus; and that, under very abnormal circumstances, the stomach, by itself, cannot expel its contents. They by no means show that in ordinary vomiting the stomach is passive; and, on the other hand, there are good reasons for believing the contrary.

It is true that facts are wanting to demonstrate with certainty this action of the stomach in vomiting; but some of the cases of fistulous opening into the organ appear to support the belief that it does take place; and the analogy of the case of the stomach with that of the other hollow viscera, as the rectum and bladder, may be also cited in confirmation.

The muscles concerned in the act of vomiting, are chiefly and primarily those of the abdomen; the diaphragm also acts, but usually not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the diaphragm; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomiting) is fixed, and presents an unyielding surface against which the stomach may be pressed. The diaphragm is, therefore, as a rule passive, during the actual expulsion of the contents of the stomach. But there are grounds for believing that sometimes this muscle actively contracts, so that the stomach is, so to speak, squeezed between the descending diaphragm and the retracting abdominal walls.

Some persons possess the power of vomiting at will, without applying any undue irritation to the stomach, but simply by a voluntary effort. It seems also, that this power may be acquired by those who do not naturally possess it, and by continual practice may become a habit. There are cases also of rare occurrence in which persons habitually swallow their food hastily, and nearly unmasticated, and then at their leisure regurgitate it, piece by piece, into their mouth, remasticate, and again swallow it, like members of the ruminant order of Mammalia.

The various nerve-actions concerned in vomiting are governed by a nerve-centre situate in the medulla oblongata.

The sensory nerves are the fifth, glosso-pharyngeal and vagus principally; but, as well, vomiting may occur from stimulation of sensory nerves from many organs, e.g., kidney, testicle, &c. The centre may also be stimulated by impressions from the cerebrum and cerebellum, so-called central vomiting occurring in disease of those parts. The efferent impulses are carried by the phrenics and other spinal nerves.

THE INTESTINES.

The Intestinal canal is divided into two chief portions, named from their differences in diameter, the (I.) small and (II.) large intestine (fig. 164). These are continuous with each other, and communicate by means of an opening guarded by a valve, the ileo-cæcal valve, which allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction.

I. The Small Intestine.-The Small Intestine, the average length of which in an adult is about twenty feet, has been divided, for convenience of description, into three portions, viz., the duodenum, which extends for eight or ten inches beyond the pylorus ; the jejunum, which forms two-fifths, and the ileum, which forms. three-fifths of the rest of the canal.

Structure. The small intestine, like the stomach, is constructed of four principal coats, viz., the serous, muscular, sub-mucous, and

mucous.

(1.) The serous coat, formed by the visceral layer of the peritoneum, and has the structure of serous membranes in general.

(2.) The muscular coats consist of an internal circular and an external longitudinal layer: the former is usually considerably

the thicker. Both alike consist of bundles of unstriped muscular tissue supported by connective tissue. They are well provided with lymphatic vessels, which form a set distinct from those of the mucous membrane.

Between the two muscular coats is a nerve-plexus (Auerbach's plexus, plexus myentericus) (fig. 189), similar in structure to Meissner's (in the submucous tissue), but with more numerous ganglia. This plexus regulates the peristaltic movements of the muscular coats of the intestines.

(3.) Between the mucous and muscular coats, is the submucous coat, which consists of connective tissue, in which numerous blood vessels and lymphatics ramify. A fine plexus, consisting mainly of non-medullated nerve-fibres, Meissner's plexus, with ganglion cells at its nodes, occurs in the submucous tissue from the stomach to the anus. From the position of this plexus and the distribution of its branches, it seems highly probable that it is the local centre for regulating the calibre of the blood-vessels supplying the intestinal mucous membrane, and presiding over the processes of secretion and absorption.

Fig. 190.-Horizontal section of a small fragment of the mucous membrane, including one entire crypt of Lieberkühn and parts of several others: a, cavity of the tubular glands or crypts; b, one of the lining epithelial cells; c, the lymphoid or retiform spaces, of which some are empty, and others occupied by lymph cells, as at d.

:

(4.) The mucous membrane is the most important coat in relation to the function of digestion. The following structures, which enter into its composition, may now be successively described :the valvula conniventes; the villi; and the glands. The general structure of the mucous membrane of the intestines resembles that of the stomach (p. 282), and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (fig. 190, c and d) enters largely into its construction; and on its deep surface is the muscularis mucose (m m, fig. 191), the fibres of which are arranged in two layers: the outer longitudinal and the inner circular.

Valvula Conniventes.-The valvulæ conniventes (fig. 192) commence in the duodenum, about one or two inches beyond the

pylorus, and becoming larger and more numerous immediately beyond the entrance of the bile duct, continue thickly arranged and well developed throughout the jejunum; then, gradually diminishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inwards of the mucous membrane; the crescentic,

nearly circular, folds thus formed being arranged transversely to the axis of the intestine, and each individual fold seldom extending around more than or of the bowel's circumference. Unlike the rugæ in the œsophagus and stomach, they do not disappear on distension of the canal. Only an imperfect notion of their natural position and function can be obtained by looking at them after the intestine has been laid open in the usual manner. To understand them aright, a piece of gut should be distended either with air or alcohol, and not opened until the tissues have become hardened. On then making a section it will be seen that, instead of disappearing, they stand out at right angles to the general surface of the mucous membrane (fig. 192). Their functions are (1) that they offer a largely increased surface for secretion and absorption, and (2) that they prevent the too rapid passage of the very liquid products of gastric digestion, immediately after their escape from the stomach, and (3), by their projection, and consequent interference with an uniform and untroubled current of the intestinal contents, that they assist in the more perfect mingling of the latter with the secretions poured out to act on them.

Fig. 191.-Vertical section through portion of small intestine of dog. v, two villi showing e, epithelium; g, goblet cells. The free surface is seen to be formed by the "striated basilar border," while inside the villus the adenoid tissue and unstriped muscle-cells are seen; f, Lieberkühn's follicles; m m, muscularis mucosa, sending up fibres between the follicles into the villi; sm, submucous tissue; containing (gm), ganglion cells of Meissner's plexus. (Schofield.)