CHAPTER VIII.

THE DIGESTIVE TRACT.

THE mouth is lined with mucous membrane, composed of stratified epithelium, upon the surface of which open the excretory ducts of numerous branched tubular glands. At the junction of the epithelium and the submucous tissue an irregular outline is presented, the projections of submucous fibrous tissue being called papillæ.

The tongue consists of a mass of voluntary musclefibers, running in various directions, surrounded by a layer of areolar tissue (the submucosa), and covered with stratified epithelium (the mucosa).

The muscular portion is made up of the geniohyoglossus, lingualis, and styloglossus muscles, and presents fibers running longitudinally, transversely, and vertically.

The submucous coat at its outer surface is thrown into marked papillæ, which are covered closely by epithelium.

Three types of papillæ are recognized:

1. Filiform papillæ, by far the most numerous, are narrow, conoid projections, which may have at their outer surface a number of smaller projections, called secondary papillæ.

2. Fungiform papillæ, not. quite so numerous as the former, are larger, their bases are somewhat con

stricted by the epithelium, and they contain secondary papillæ on their sides as well as at the outer surface.

3. The circumvallate papillæ are the largest and broadest of the three types. They occur in smaller numbers, and are usually found at the posterior portion of the tongue. They contain secondary papillæ at the top. At the sides, the epithelium dips down to form furrows or crypts, which make the papillæ appear to stand out from the surrounding tissue.

Embedded in the epithelium at the sides of the circumvallate papillæ are the taste-buds.

Fig. 26.-Tongue: a, Filiform papillus; b, filiform papillus with secondary papillæ; c, fungiform papillus; d, circumvallate papillus; e, taste-bud.

The taste-buds are the seat of the sense of taste, and consist of several long, spindle-shaped, neuro-epithelial cells (the gustatory cells), with elongated nuclei, which are surrounded by a layer of slightly flattened cells (the cortical or tegment cells), the whole forming small flask-shaped bodies, which are usually grouped side by side, vertical to the epithelial surface.

Besides the foregoing structures, the tongue contains a vertical partition, mostly of fibrous tissue (the septum linguale), which divides it into laterally symmetric

halves. Numerous mucous and serous glands also occur in the submucous tissue.

The tongue is richly supplied with lymphatics, the larger vessels being situated deeply, the superficial ones forming a plexus under the mucosa and sending small branches up into the papillæ. Numerous lymph-follicles and areas of diffuse adenoid tissue occur in connection with the lymphatics, being especially numerous at the root of the tongue.

The blood supply is very abundant, plexuses of small arterioles occurring throughout the submucosa and sending branches into the papillæ and around the numerous glands.

The mucous membrane is richly supplied with nerves, both sensory and taste. They are mostly medullated fibers from the glossopharyngeal and lingual nerves. Numerous ganglia also occur along the course of the former.

THE TEETH.

Although the teeth vary considerably in shape and in general appearance, the same general structure is found in all.

The outer or exposed part of the tooth is called the crown; the portion embedded in the socket is called the fang. Between these, at about the edge of the gum, is a slightly constricted portion-the neck.

The main bulk of the tooth is composed of the dentin, or ivory. This gives the form to the tooth, and incloses the pulp-cavity, which contains the pulp.

At the crown the dentin is covered by the enamel. At the fang it is covered by the cementum, except at the apex, where a minute canal penetrates to the pulpcavity.

The dentin is composed of a hard, white, apparently

homogeneous matrix, in which are numerous small (25 microns = 1000 of an inch) canals the dentinal tubules. These dentinal tubules originate in the pulpcavity, and run a wavy course, more or less at right angles to the inner surface, through the dentin, and terminate at the juncture of the dentin and the enamel or cementum in irregular dilatations, called the inter

Fig. 27.-Tooth: a, Enamel; b, dentin; c, pulp-cavity; d, junction of enamel and cementum; e, cementum; f, alveolar periosteum.

globular spaces. Along their course many communicating branches occur, uniting the dentinal tubules. That portion of the matrix which forms the wall of the tubules being harder and denser than the main body, it has received the special name of dentinal sheath.

The enamel consists of closely arranged columns (the enamel prisms), running vertically to the free

spaces. In structure the serous membranes are composed of a single layer of thin, irregular, polygonal cells, placed edge to edge and united by cement substance, which rest upon a rather thin sheet of fibrous tissue. At irregular intervals small openings occur (the stomata), which are the mouths of lymphatics. Occasionally, a place is found at which the endothelial cells do not "match up" well. These spaces thus

formed are filled up by cement substance, and are called the pseudostomata.

The principal serous membranes of the body are the pleura, pericardium, and peritoneum, and the synovial membranes of the tendons and joints.

Structurally, the internal lining of heart, blood-vessels, and lymphatics would also come under this head.