the chief substance of the salivary secretion, i. e., the ferment ptyalin. In the case of the submaxillary gland of the dog, at any rate, the sympathetic nerve-fibres appear to have to do with the first stage of the process, and when stimulated the protoplasm is extremely active in manufacturing the granules, whereas the chorda tympani is concerned in the production of the second act, the actual discharge of the materials of secretion, together with a considerable amount of fluid, the latter being an actual secretion by the protoplasm, as it ceases to occur when atropin has been subcutaneously injected.

In the mucous-secreting gland, the changes in the cells during secretion have been already spoken of (p. 265). They consist in the gradual secretion by the protoplasm of the cell of a substance called mucigen, which is converted into mucin, and discharged on secretion into the canal of the alveoli. The mucigen is, for the most part, collected into the inner part of the cells during rest, pressing the nucleus and the small portion of the protoplasm which remains, against the limiting membrane of the alveoli.

The process of secretion in the salivary glands is identical with that of glands in general; the cells which line the ultimate branches of the ducts being the agents by which the special constituents of the saliva are formed. The materials which they have incorporated with themselves are almost at once given up again, in the form of a fluid (secretion), which escapes from the ducts of the gland; and the cells, themselves, undergo disintegration,— again to be renewed, in the intervals of the active exercise of their functions. The source whence the cells obtain the materials of their secretion, is the blood, or, to speak more accurately, the plasma, which is filtered off from the circulating blood into the interstices of the glands as of all living textures.

THE PHARYNX.

That portion of the alimentary canal which intervenes between the mouth and the oesophagus is termed the Pharynx (fig. 164) It will suffice here to mention that it is constructed of a series of three muscles with striated fibres (constrictors), which are covered by a thin fascia externally, and are lined internally by a strong fascia (pharyngeal aponeurosis), on the inner aspect of which is areolar (submucous) tissue and mucous membrane, con

tinuous with that of the mouth, and, as regards the part concerned in swallowing, is identical with it in general structure. The epithelium of this part of the pharynx,

Fig. 180.-Lingual follicle or crypt. a, involution of mucous membrane with its papille; b, lymphoid tissues, with several lymphoid sacs. (Frey.)

like that of the mouth, is stratified and squamous.

The pharynx is well supplied with mucous glands (fig. 182).

The Tonsils.

Between the anterior and posterior arches of the soft palate are situated the Tonsils, one on each side. A tonsil consists of an elevation of the mucous membrane presenting 12 to 15 orifices, which lead into crypts or recesses, in the walls of which are placed nodules of adenoid or lymphoid tissue (fig. 181). These nodules are enveloped in a less dense adenoid tissue

Fig. 181.-Vertical section through a crypt of the human tonsil. a, entrance to the crypt, which is divided below by the elevation which does not quite reach the surface; b, stratified epithelium; c, masses of adenoid tissue; d, mucous glands cut across; e, fibrous capsule. Semidiagrammatic. (V. D. Harris.)

which reaches the mucous surface.

The surface is covered with

stratified squamous epithelium, and the subepithelial or mucous

membrane proper may present rudimentary papillæ formed of adenoid tissue. The tonsil is bounded by a fibrous capsule (fig. 181, e). Into the crypts open the ducts of numerous mucous glands.

The viscid secretion which exudes from the tonsils serves to lubricate the bolus of food as it

passes them in the second part of the act of deglutition.

THE ESOPHAGUS OR GULLET.

The Esophagus or Gullet (fig. 164), the narrowest portion of the alimentary canal, is a muscular and mucous tube, nine or ten inches in length, which extends from the lower end of the pharynx to the cardiac orifice of the stomach.

B..

C...

Structure. - The oesophagus is made up of three coats-viz., the outer, muscular; the middle, submucous; and the inner, mucous. The muscular coat (fig. 183, g and i), is covered externally by a varying amount of loose fibrous tissue. It is composed of two layers of fibres, the outer being arranged longitudinally, and the inner circularly. At the upper part of the oesophagus this coat is made up principally of striated muscle fibres, as they are continuous with the constrictor muscles of the pharynx; but lower down the unstriated fibres become more and more numerous, and towards the end of the tube form the entire coat. The muscular coat is connected with the mucous coat by a more or less developed layer of areolar tissue, which forms the submucous coat (fig. 183, ƒ), in which is contained in the lower half or third of the tube many mucous glands, the ducts of which, passing through the mucous membrane (fig. 183, c) open on its surface. Sepa

Fig. 182.-Section of a mucous gland from the tongue. A, opening of the duct on the free surface; Č, basement membrane with nuclei; B, flattened epithelial cells lining duct. The duct divides into several branches, which are convoluted and end blindly, being lined throughout by columnar epithelium. D, lumen of one of the tubuli of the gland. x 90. (Klein and Noble Smith.)

rating this coat from the mucous membrane proper is a welldeveloped layer of longitudinal, unstriated muscle (d), called the muscularis mucosa. The mucous membrane is composed of a closely felted meshwork of fine connective tissue, which, towards

Fig. 183.-Longitudinal section of the esophagus of a dog towards the lower end. a, stratified epithelium of the mucous membrane; b, mucous membrane proper; c, duct of mucous gland; d, muscularis mucosa; e, mucous glands; f, submucous coat; g, circular muscular layer; h, intermuscular layer, in which is contained the ganglion cells of Auerbach; i, longitudinal muscular layer; k, outside investment of fibrous tissue. Semidiagrammatic. (V. D. Harris.)

the surface, is elevated into rudimentary papillæ. It is covered with a stratified epithelium, of which the most superficial layers are squamous. The epithelium is arranged upon a basement membrane.

In newly-born children the mucous membrane exhibits, in many parts, the structure of lymphoid tissue (Klein).

Blood- and lymph-vessels, and nerves, are distributed in the walls of the œsophagus. Between the outer and inner layers of the muscular coat, nerve-ganglia of Auerbach are also found.

DEGLUTITION OR SWALLOWING.

When properly masticated, the food is transmitted in successive portions to the stomach by the act of deglutition or swallowing. This, for the purpose of description, may be divided into three acts. In the first, particles of food collected to a morsel are made to glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces; in the second, the morsel is carried through the pharynx; and in the third, it reaches the stomach through the œsophagus. These three acts follow each other rapidly. (1.) The first act may be voluntary, although it is usually performed unconsciously; the morsel of food, when sufficiently masticated, being pressed between the tongue and palate, by the agency of the muscles of the former, in such a manner as to force it back to the entrance of the pharynx. (2.) The second act is the most complicated, because the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the anterior arches of the palate, it is moved onwards by the movement of the tongue backwards, and by the muscles of the anterior arches contracting on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried forwards under the base of the tongue, the epiglottis is pressed over the upper opening of the larynx, and the morsel glides past it; the closure of the glottis being additionally secured by the simultaneous contraction of its own muscles: so that, even when the epiglottis is destroyed, there is little danger of food or drink passing into the larynx so long as its muscles can act freely. At the same time, the raising of the soft palate, so that its posterior edge. touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly inwards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the œsophagus. (3.) In the third act, in which the food passes through the œsophagus, every part of that tube, as it receives the morsel and is dilated by it, is stimulated to contract: hence an undulatory contraction of the œsophagus, which is easily observable in horses while drinking, proceeds rapidly