along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive contraction of the œsophagus is slow, and attended with pain. Division of both pneumogastric nerves paralyses the contractile power of the œsophagus, and food accordingly accumulates in the tube. The second and third parts of the act of deglutition are involuntary. Nerve Mechanism.-The nerves engaged in the reflex act of deglutition are:-sensory, branches of the fifth cerebral supplying the soft palate; glosso-pharyngeal, supplying the tongue and pharynx; the superior laryngeal branch of the vagus, supplying the epiglottis and the glottis; while the motor fibres concerned are-branches of the fifth, supplying part of the digastric and mylo-hyoid muscles, and the muscles of mastication; the facial, supplying the levator palati; the glosso-pharyngeal, supplying the muscles of the pharynx; the vagus, supplying the muscles of the larynx through the inferior laryngeal branch, and the hypoglossal, the muscles of the tongue. The nerve-centre by which the muscles are harmonised in their action, is situate in the medulla oblongata. In the movements of the oesophagus, the ganglia contained in its walls, with the pneumo-gastrics, are the nerve-structures chiefly concerned.

It is important to note that the swallowing both of food and drink is a muscular act, and can, therefore, take place in opposition to the force of gravity. Thus, horses and many other animals habitually drink up-hill, and the same feat can be performed by jugglers.

THE STOMACH.

In man and those Mammalia which are provided with a single stomach, it consists of a dilatation of the alimentary canal placed between and continuous with the œsophagus, which enters its larger or cardiac end on the one hand, and the small intestine, which commences at its narrowed end or pylorus, on the other. It varies in shape and size according to its state of distension.

The Ruminants (ox, sheep, deer, &c.) possess very complex stomachs; in most of them four distinct cavities are to be distinguished (fig. 184).

1. The Paunch or Rumen, a very large cavity which occupies the cardiac end, and into which large quantities of food are in the first instance swallowed with little or no mastication. 2. The Reticulum, or Honeycomb stomach, so called from the fact that its mucous membrane is disposed in a

number of folds enclosing hexagonal cells. 3. The Psalterium, or Manyplies, in which the mucous membrane is arranged in very prominent longitudinal folds. 4. Abomasum, Reed, or Rennet, narrow and elongated, its mucous membrane being much more highly vascular than that of the other divisions. In the process of rumination small portions of the contents of the rumen and reticulum are successively regurgitated into the mouth, and there thoroughly masticated and insalivated (chewing the cud): they are then again swallowed, being this time directed by a groove (which in the figure is seen running from the lower end of the oesophagus) into the many plies, and thence into the abomasum. It will thus be seen that the first two stomachs

Fig. 184.-Stomach of a sheep. a, cesophagus; Ru, rumen; Ret, reticulum; Ps, psalterium, or manyplies; A, abomasum; Du, duodenum; g, groove from oesophagus to psalterium. (Huxley.)

(paunch and reticulum) have chiefly the mechanical functions of storing and moistening the fodder: the third (manyplies) probably acts as a strainer, only allowing the finely divided portions of food to pass on into the fourth stomach, where the gastric juice is secreted and the process of digestion carried on. The mucous membrane of the first three stomachs is lowly vascular, while that of the fourth is pulpy, glandular, and highly vascular.

In some other animals, as the pig, a similar distinction obtains between the mucous membrane in different parts of the stomach.

In the pig the glands in the cardiac end are few and small, while towards the pylorus they are abundant and large.

A similar division of the stomach into a cardiac (receptive) and a pyloric (digestive) part, foreshadowing the complex stomach of ruminants, is seen in the common rat, in which these two divisions of the stomach are distinguished, not only by the characters of their lining membrane, but also by a well-marked constriction.

In birds the function of mastication is performed by the stomach (gizzard) which in granivorous orders, e.g., the common fowl, possesses very powerful muscular walls and a dense horny epithelium.

Structure.-The stomach is composed of four coats, called respectively-an external or (1) peritoneal, (2) muscular, (3) submucous, and (4) mucous coat; with blood-vessels, lymphatics, and nerves distributed in and between them.

(1) The peritoneal coat has the structure of serous membranes

in general. (2) The muscular coat consists of three separate layers or sets of fibres, which, according to their several directions, are named the longitudinal, circular, and oblique. The longitu dinal set are the most superficial: they are continuous with the longitudinal fibres of the œsophagus, and spread out in a diverging manner over the cardiac end and sides of the stomach. They extend as far as the pylorus, being especially distinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set are the circular or transverse fibres, which more or less completely encircle all parts of the stomach; they are most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the pylorus. These fibres are not simple circles, but form double or figure-of-8 loops, the fibres intersecting very obliquely. The next, and consequently deepest set of fibres, are the oblique, continuous with the circular muscular fibres of the œsophagus, and having the same double-looped arrangement that prevails in the preceding layer: they are comparatively few in number, and are placed only at the cardiac orifice and portion of the stomach, over both surfaces of which they are spread, some passing obliquely from left to right, others from right to left, around the cardiac orifice, to which, by their interlacing, they form a kind of sphincter, continuous with that around the lower end of the œsophagus. The muscular fibres of the stomach and of the intestinal canal are unstriated, being composed of elongated, spindle-shaped fibre-cells.

(3) and (4) The mucous membrane of the stomach, which rests upon a layer of loose cellular membrane, or submucous tissue, is smooth, level, soft, and velvety; of a pale pink colour during life, and in the contracted state thrown into numerous, chiefly longitudinal, folds or rugæ, which disappear when the organ is distended.

The basis of the mucous membrane is a fine connective tissue, which approaches closely in structure to adenoid tissue; this tissue supports the tubular glands of which the superficial and chief part of the mucous membrane is composed, and passing up between them assists in binding them together. Here and there are to be found in this coat, immediately underneath the glands, masses of adenoid tissue sufficiently marked to be termed by some lymphoid follicles. The glands are separated from the rest of the mucous membrane by a very fine homogeneous basement membrane. At the deepest part of the mucous membrane are two layers

(circular and longitudinal) of unstriped muscular fibres, called the muscularis mucosa, which separate the mucous membrane from the scanty submucous tissue.

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When examined with a lens, the internal or free surface of the stomach presents a peculiar honeycomb appearance, produced by shallow polygonal depressions, the diameter of which varies generally from 2th to th of an inch; but near the pylorus is as much as th of an inch. They are separated by slightly elevated ridges, which sometimes, especially in certain morbid states of the stomach, bear minute, narrow vascular processes, which look like villi, and have given rise to the erroneous supposition that the stomach has absorbing villi, like those of the small intestines. In the bottom of these little pits, and to some extent between them, minute openings are visible, which are the orifices of the ducts of perpendicularly arranged tubular glands (fig. 185), imbedded side by side in sets or bundles, on the

Fig. 185. From a vertical section through the mucous membrane of the cardiac end of stomach. Two peptic glands are shown with a duct common to both, one gland only in part. a, duct with columnar epithelium becoming shorter as the cells are traced downward; n, neck of gland tubes, with central and parietal or so-called peptic cells; b, fundus with curved cæcal extremity-the parietal cells are not so numerous here. × 400. (Klein and Noble Smith.)

surface of the mucous membrane, and composing nearly the whole structure.

Gastric Glands.--Of these there are two varieties, (a) Peptic, (b) Pyloric or Mucous.

(a) Peptic glands are found throughout the whole of the stomach except at the pylorus. They are arranged in groups of four or five, which are separated by a fine connective tissue. Two or three tubes often open into one duct, which forms about a third of the whole length of the tube and opens on the surface. The ducts are lined with columnar epithelium. Of the gland tube proper, i.e., the part of the gland below the duct, the upper third is the neck and the rest the body.

-b

The neck is narrower than the body, and is lined with granular cubical cells which are continuous with the columnar cells of the duct. Between these cells and the membrana propria of the tubes, are large oval or spherical cells, opaque or granular in appearance, with clear oval nuclei, bulging out the membrana propria; these cells are called peptic or parietal cells. They do not form a continuous layer. The body, which is broader than the neck and terminates in a blind extremity or fundus near the muscularis mucosæ, is lined by cells continuous with the cubical or central cells of the neck, but longer, more columnar and more transparent. In this part are a few parietal cells of the same kind as in the neck (fig. 185).

Fig. 186. Transverse section through lower part of peptic glands of a cat. a, peptic cells; b, small spheroidal or cubical cells; c, transverse section of capillaries. (Frey.)

As the pylorus is approached the gland ducts become longer, and the tube proper becomes shorter, and occasionally branched at the fundus.

(b) Pyloric Glands.-These glands (fig. 187) have much longer ducts than the peptic glands. Into each duct two or three tubes open by very short and narrow necks, and the body of each tube is branched, wavy, and convoluted. The lumen is very large. The ducts are lined with columnar epithelium, and the neck and body with shorter and more granular cubical cells, which correspond with the central cells of the peptic glands. During secretion the cells become, as in the case of the peptic glands, larger and the granules restricted to the inner zone of the cell. As they approach the duodenum the pyloric glands become larger, more