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neurones found in the sublingual and submaxillary ganglia, and not in the glands, as generally stated by writers. The increase of secretion from the submaxillary and sublingual glands on direct or indirect stimulation of the chorda tympani is due, therefore, not to a direct stimulation of the gland-cells by the fibers of this nerve, but to a stimulation of the sympathetic neurones of the sublingual and submaxillary ganglia, the neuraxes of which convey the impulse to the gland-cells. These glands have a further nerve supply from the superior cervical ganglia of the cervical sympathetic. The neuraxes of sympathetic neurones, the cell-bodies of which are situated in the superior cervical ganglia, accompany the blood-vessels to the sublingual and submaxillary glands; their mode of termination is, however, not as yet determined. The cell-bodies of the sympathetic neurones here in question are surrounded by end-baskets of nerves which leave the spinal cord through the second, third, and fourth. dorsal spinal roots. The blood-vessels of the salivary glands are also richly supplied with vasomotor nerves, the neuraxes of sympathetic neurones, which terminate on the muscle-cells of their walls. The nerve supply of the parotid glands is, in the main, like that of the other salivary glands here described, although it has not been worked out with the same detail. The cell-bodies of the sympathetic neurones, the neuraxes of which innervate the gland-cells, are, it would appear, situated in the otic ganglia. The nerve-ending in the smaller glands of the mouth is similar to that given for the salivary glands, as has been shown by Retzius and other observers.

It is very probable that the cell-bodies of the sympathetic neurones, the neuraxes of which innervate the glands of the tongue, are situated in the small sympathetic ganglia found on the lingual branches of the glossopharyngeal and lingual nerves.

B. THE PHARYNX AND ESOPHAGUS.

The mucous membrane of the pharynx and esophagus is similar in structure to that of the oral cavity.

The epithelium is of the stratified squamous variety, and also contains prickle cells and keratohyalin. (See Skin.) A stratified ciliated epithelium is present only in the fornix in the region of the posterior nares. The area covered by this type of epithelium is more extensive in the fetus and new-born, and extends over the whole nasopharyngeal vault. In the human embryo the superficial epithelial cells of the esophagus possess cilia up to the thirtysecond week (Neumann, 76). The papillæ of the mucosa are loosely arranged and are in the form of slender cones. The mucosa of the pharynx contains diffuse adenoid tissue rich in cells which in some places forms accessory tonsils (vid. p. 225). There are but few mucous glands in the submucous tissue of the esophagus, but when present they contain well-marked demilunes. In

man the ducts of these glands do not reach the surface between the connective-tissue papillæ, as in the external skin, but pass up through them into the epithelium and thus to the surface. A layer consisting of nonstriated muscle-fibers, the muscularis mucosa, the majority of the cells of which show a longitudinal arrangement, is found between the mucosa and submucosa in the esophagus, but not in the pharynx.

The external muscular coat of the pharynx is made up of transversely striated muscle-fibers, arranged in a complicated manThis tissue extends downward to about the middle of the

ner.

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esophagus, in which it consists of an outer longitudinal and an inner circular layer. In the lower half of the esophagus nonstriated muscle-fibers alone are present. There is no sharply defined line of demarcation between the two types of muscular tissue, as the fibers of the unstriped variety penetrate for some distance upward into the substance of the striated muscle, giving the tissue here a mixed character.

C. THE STOMACH AND INTESTINE

1. GENERAL STRUCTURE OF THE INTESTINAL MUCOUS
MEMBRANE.

The mucous membrane of the stomach and intestine, unlike that of the esophagus and oral cavity, possesses an epithelium of the simple columnar variety with elongated cells (about 22 μ in

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Fig. 197.-Part of section of human esophagus, showing duct of mucous gland; X 120.

height). In the intestine the epithelium shows a well-marked striated cuticular border, striated protoplasm in the outer ends of the cells, extending to the immediate vicinity of the nuclei, which are situated in the basal portions of the cells. The basal portion of each cell consists of nonstriated protoplasm, ending in a longer or shorter process which extends to the basement membrane, or possibly

even penetrates it. The epithelial cells have the power of producing mucus, a phenomenon which, in the normal condition, rarely embraces whole arcas of epithelium; these cells (goblet cells) are usually surrounded by others which are unchanged (for details about goblet cells see General Histology, p. 81). Throughout the entire intestinal tract the epithelium forms simple, branched, and compound tubular and alveolar glands. These are depressions lying in the mucosa, and only in the duodenum extend beyond it into the sub

mucosa.

The mucosa consists of adenoid tissue, containing relatively few cells. It fills the interstices between the glands, and often forms a thin but continuous layer (granular layer of F. P. Mall) below the glands. It is therefore obvious that the development of the mucosa is inversely proportionate to the number and the density of arrangement of the glands; when the latter are present in large numbers, as, for instance, in the stomach, the mucosa is reduced to a minimum. In the small intestine it forms not only the permanent folds, but also certain finger-like elevations known as villi, which are covered with epithelium and project into the lumen of the intestine, thus increasing to a considerable extent the surface area of the mucous membrane. In the mucosa are found small nodules of adenoid tissue. These are spoken of as lenticular glands when occurring in the stomach, as solitary glands when found in the upper portion of the small intestine and in the large intestine. In the lower portion of the small intestine they are grouped to form the agminated glands, or Peyer's patches, which, when large, extend into the submucosa. Beneath the stratum proprium is a layer consisting of two or three strata of unstriped muscle-fibers, the muscularis mucosæ. As a rule, it is composed of an inner circular and an outer longitudinal layer. This arrangement is interrupted only where the larger glands and follicles penetrate into the submucosa. The epithelium with the glands, the mucosa with its lymph-nodules, and the muscularis mucosa form together the mucous membrane, or tunica mucosa.

Below the mucous membrane is the connective-tissue submucosa. This is characterized by its loose structure, and consequently affords considerable mobility to the mucous membrane. In the small intestine it forms a large number of permanent transverse folds known as valvula conniventes (Kerkring). In the submucosa of the duodenum occur the secreting portions of Brunner's glands (glandulæ duodenales), and in the small intestine the larger lymph-nodes and Peyer's patches.

External to the submucosa is the muscular coat, which generally consists of two layers of unstriped muscle-tissue. The inner layer is composed of circular fibers (stratum circulare); the outer layer, of longitudinal fibers (stratum longitudinale). In the colon the longitudinal layer forms definite bands, the tenia coli. In some regions the circular fibers are also considerably reinforced, particularly in

the plice sigmoidea which lie between the tæniæ coli. At these points the longitudinal layer also is thickened. In the rectum the circular fibers form the internal sphincter ani muscle. In the stomach a third layer is added to the two already mentioned, with fibers running obliquely. It lies internal to the circular fibers, but does not form a continuous layer.

According to Legge, elastic fibers are present throughout the entire digestive tract of all adult mammalia and vary only in minor details in the different species. In regions in which the tunica muscularis is prominent the elastic fibers attain a considerable size. There is also a difference in their development in carnivora and herbivora. In general, they form a dense network, present not only in the serous layer, but also in the submucosa and beneath the epithelium. These fibers preserve the elasticity of the intestinal walls and resist any hyperextension of the glands and follicles.

The intestine is covered externally by the peritoneum, forming the serous coat, which consists of an inner, very thin connectivetissue layer (subserosa) and an outer layer of mesothelial cells.

2. THE STOMACH.

The general structure of the gastric mucous membrane is essen

tially the same as that of the intestinal canal.

It presents, however,

Basement membrane.

depressions known as gastric crypts, due to an infolding of the epithelium into which the gastric glands open. In the fundus the crypts attain a depth of from one-fifth to one-sixth the thickness of the mucous membrane. In the pylorus they are deeper, many of them here extending through half the mucous membrane and some even reaching the muscularis mucosa. The epithelium of the crypts and that of the folds between them is composed of long, slender cells, with basally situated nuclei. That portion of the cellbody near its free margin contains very little protoplasm, but is, on the other hand, rich in paraplasm; the region of the cell containing the nucleus possesses more protoplasm. This part of the cell extends downward in a curved process of diminishing size, which assumes a position parallel to the corresponding parts of the neighboring cells, and finally penetrates the basement membrane. Into a single gastric crypt of the human fundus empty from three to seven gastric glands. Each gland consists of a simple tube, from

[graphic]

Fig. 198.-Epithelium of human stomach, covering the fold of mucosa between

two gastric crypts; X 700. Technic No.

241.

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