With a high power it is easy to observe the structure of the

several parts, and to see the odontoblasts lining what is to be the

FIG. 247.-T.S. Lower Jaw of New-Born Dog, X 40. The dental sac is shown only in the
left side. The tissues originating from connective tissue are shown on the left, and
those of epithelial origin on the right.

pulp-cavity. If the latter happen to be partially detached, their

processes-fibres of Tomes--may be seen partially withdrawn from the dentinal tubules in which they lay.

ESOPHAGUS.

The Esopagus consists from within outwards of

(1.) Mucous coat, composed of stratified squamous epithelium, into which project small simple papillæ from the corium or connective-tissue basis of the membrane. At the outer part of the corium is a narrow layer of smooth muscular fibres-muscularis mucosæ-arranged for the most part longitudinally.

(2.) Submucous coat, consisting of connective tissue and the larger blood-vessels and some nerves. In those animals (e.g., dog) in which glands occur, the acini of the

glands lie in the submucous coat, so that their ducts have to perforate the muscularis mucosa and traverse the mucous membrane before they open on the surface of the epithelium.

(3.) Muscular coat. This will vary

with the animal used. In man this coat in the upper third of the oesophagus is composed of striated muscular fibre, the lower two-thirds of smooth muscle. The outer layer of fibres runs longitudinally, the inner circularly.

(4.) Fibrous coat, composed of fibrous tissue.

4. The Esophagus.- Cut out a piece of the œsophagus of a dog or cat-2 cm. in length and harden it in equal parts

MM

of chromic acid (per cent.) and spirit, or FIG. 248.-T.S. Small Part of Müller's fluid (fourteen days), or mercuric chloride, and then in alcohol.

Make

transverse sections; stain one in picrocarmine and mount it in Farrant's solution, and another in logwood and mount

Esophagus of Dog. E. Epithelium; M.M. Muscularis mucosa; G. Glands; C. Circular, and L. External or longitudinal muscular coat; F. Fibrous layer. Müller's fluid, picro-carmine.

it in balsam. Perhaps even more instructive sections are obtained from a small animal, such as a rat, where the whole œsophagus (with trachea) can be stained "in bulk" in borax-carmine. Cut sections of both tubes to show their structure and relations.

(a.) (L and H) The circular tube has several coats, the innermost one being thrown into folds. The mucous membrane covered by stratified squamous epithelium, under this the connective tissue

with small simple papillæ. In the deeper part of the mucous membrane are several layers of non-striped muscle, the muscularis mucose (248, M.M).

(b.) Outside this is loose submucous connective tissue with a few blood-vessels, and in the dog and some other animals the acini of mucous glands. The ducts of the latter traverse the coats lying internal to the glands, and open on the inner surface by funnelshaped openings.

(c) Outside this, again, is the muscular coat, which varies in the upper and lower parts of the tube, and also with the animal examined. In the upper part there is striped muscle, in the lower part two layers of non-striped muscle, an inner circular and an outer longitudinal. Between the two layers may be seen ganglionic cells of Auerbach's plexus.

(.) Outside all is the fibrous coat or adventitia, composed of coarser connective tissue, with elastic fibres and blood-vessels.

For the epithelial cells lining the oesophagus see Lesson IV. 5. It is to be remembered that there are very great differences as regards the presence of glands in the oesophagus. Some animals have a considerable number-e.g., dog-and others very few.

ADDITIONAL EXERCISE.

5. Other Methods (Esophagus). -Very good results are obtained by hardening in absolute alcohol containing methyl-green, the gland-cells being thereby sharply defined (Rubeli).1 Also double stain with borax-carmine (extract with acid-alcohol), then alcohol, and stain again with iodine-green (twenty-four hours), extract with alcohol, embed, and cut in paraffin. The mucous membrane of some animals (e.g., pig) contains lymph follicles, which can readily be detected in a part of the tube stained in bulk in borax-carmine.

LESSON XXIII.

THE SALIVARY GLANDS AND PANCREAS.

THE SALIVARY GLANDS.

ALL these glands have not the same structure, hence it is necessary to classify them.

Mucous Salivary Glands.-The sub-maxillary and sub-lingual glands of the dog and sub-lingual of guinea-pig.

1 Zeits. f. mik. Anat., vii. p. 224, 1890.

Serous Salivary Glands. The parotid of man and mammals, and the sub-maxillary of the rabbit.

Mixed or Muco-Salivary. The human sub-maxillary, retrolingual of the dog, or sub-maxillary of the guinea-pig.

The salivary glands are compound tubular glands, i.e., the duct is branched, while the acini or alveoli the true secretory parts of the glands-are tubular in form. Each gland consists of lobes, held together by connective tissue, which forms a capsule for the whole gland and gives septa to enclose the lobes and lobules. Each lobe in turn is made up of numerous smaller lobules also held together by connective tissue, which carries the blood-vessels, nerves, lymphatics, and larger ducts. From mutual pressure the lobes and lobules are usually polygonal in shape. The main duct is made up by the convergence of ducts from the lobes-lobar ducts-while from each lobule there is a duct-lobular ducts, which unite to form lobar ducts. Each lobule is made up of a number of alveoli or acini. Each alveolus, which has a closed extremity, leads into or discharges its secretion into a fine duct or ductule, and these ductules by their union form the intralobular ducts. Practically the arrangement of the ducts is the same in all this set of glands; the differences in structure are in the alveoli. The alveoli consist of a basement membrane, which by appropriate means can be shown to consist of branched cells forming a reticulated or basket-like membrane. This is lined internally by the secretory epithelium, leaving a larger or smaller lumen in the centre, which leads into a fine duct or ductule by means of a narrow junctional piece or intermediary part or ductule, in which the epithelium is somewhat flattened. Usually several alveoli open into one intermediary tubule or ductule. The ducts with a fibrous wall are lined by a single layer of columnar epithelium, which is striated or "rodded" in its outer part, and granular towards the lumen of the tube (fig. 249); a little inwards from the centre of each cell is a nucleus.

In mucous glands the acini (35 μ in diameter) are lined by a layer of polyhedral clear cells, whose broader bases rest on the basement membrane, while their apices abut on the lumen, which is small (fig. 249). Usually in a transverse section of an acinus five or six cells are seen. The appearance of these cells varies according as a gland is at rest or in a state of activity, i.e., whether the gland is "loaded" or "charged" (resting phase), or "unloaded" or "discharged" (active phase). In a resting gland the mucous cells are clear, for the most part, while at the outer part of the cell is a flattened nucleus surrounded by a very small quantity of granular protoplasm. The clear part is traversed by a network of fibrils, which includes in its meshes mucigen. The granular matter

R

and nuclei stain readily with the ordinary dyes, while the clear part does not do so.

In some mucous glands, e.g. dog, but not in all mucous glands, here and there between the bases of some of these cells, and the basement membrane, are groups of small, granular, nucleated cells, the group having a somewhat crescentic shape; they are called demilunes or crescents of Gianuzzi (fig. 250). They stain readily with dyes, and are darkened by osmic acid, and contain two or more nuclei.

In the discharged or active gland, the acini are smaller, the lumen wider, the clear part of the cell diminished in volume, while the outer part of the cell is wider, and appears to have encroached on the clear part. The nucleus is usually spherical, and placed nearer the centre of the cell.

In serous or albuminous glands the chief differences are in the cells lining the alveoli. In serous alveoli there is but one layer of cells, and nothing corresponding to the demilunes. The cells are somewhat smaller than mucous cells; they are more granular, and stain more uniformly with dyes. The nucleus is spherical, and placed nearer the centre of the cell. The differences between active and passive phases are not so marked as in mucous acini.

During activity the cells become smaller, and the "granules" disappear from the outer part of the cell; the cells become more sharply defined, while the nuclei are large and spherical.

N.B.—In all cases examine the acini of the glands in the fresh condition.

A. Mucous Salivary Glands.-These must be prepared in several ways.

Methods. (i.) For the general structure of salivary glands :Harden small piece; for 2 or 3 days in the following mixture :3 parts 90 per cent. alcohol and 2 parts .5 per cent. chromic acid. To see the finer points-after staining-mount the sections in glycerine (Langley).

(ii.) Small pieces of a perfectly fresh dog's sub-maxillary gland are placed for an hour in 75 per cent. alcohol, then for five hours in absolute alcohol, which is then changed, and the hardening is completed in fresh absolute alcohol in twenty-four hours. Sections of the unstained gland are apt to fall to pieces, although the small pieces show the structure sufficiently well. A part of the alcoholhardened gland should be stained "in bulk" in borax-carmine, and cut in paraffin. In this way the relative position of the parts is retained.

I

(iii) Harden very small pieces in 1 per cent. osmic acid (24 hours); wash thoroughly, and complete the hardening in alcohol.