The Lungs. The lungs are developed from the two diverticula of the caudal end of the median longitudinal groove and the mesodermal tissue into which these grow. Originally single, this caudal end soon becomes bilobed and pouches out on each side into two lateral diverticula, which represent the primitive bronchi and lungs. From the first the right pulmonary diverticulum or vesicle is slightly the larger of the two. Both diverticula elongate, and almost immediately undergo a subdivision-the right into three vesicles, and the left into two vesicles-thus early indicating the three lobes of the right lung and the two lobes of the left lung. As the primitive respiratory tube lies in the median plane in the dorsal attachment of the septum transversum, the pulmonary diverticula grow laterally and dorsally into the dorsal parietal recesses, that is into the future pleural cavities, carrying before them a covering of mesoblast. From this mesoblast are derived the blood-vessels and other tissues which build up the lung, whilst the entodermal cells which form the lining membrane of the primitive respiratory tube eventually develop into the epithelial lining of the air-passages, and are embedded within the surrounding mesoderm. The main entodermal subdivisions continue to branch and re-branch, pushing their way into the pulmonary mesoblast, until the complete bronchial tree is formed.

The primary pulmonary diverticula increase in size and complexity as additional outgrowths arise by the subdivision of the enlarged terminal part of each diverticulum. Their mode of subdivision is very characteristic, and from the first the various branches are bulbous or flask-shaped at their extremities. These bifurcate, and although at first the two main subdivisions appear, in each case, of equal importance, one grows out as the continuation of the main bronchial stem-the future hyparterial bronchus whilst the other remains as a branch. When the ramification of the entodermal tubes into the lung-mesoderm is complete, the small terminal flask-shaped extremities of the various branches represent the atria of the lung.

This repeated bifurcation results, as just stated, in the formation of a main bronchus which traverses the entire length of the lung, and into which numerous secondary bronchi open. The latter, from the manner in which they arrange themselves around the main stem of the pulmonary artery, are divided into dorsal and ventral. These alternate with each other, and usually number four in each series; not infrequently the third dorsal bronchus fails to develop. In the left lung the first dorsal bronchus arises, not from the main tube as on the right side, but from the first ventral bronchus-an arrangement which probably results from the fusion on the left side of the superior and middle lobes of the left lung into one, namely, the so-called lobus superior of the adult left lung.

The secondary bronchi elongate, and give rise to the tertiary bronchi, and these in turn to lesser bronchi, and so on down to the terminal bronchi, with their atria, air-sacs, and air-cells of the lung-unit. At first the lung-unit is devoid of air-cells, but between the sixth month and full term the alveolar saccules and air-cells make their appearance on the alveolar ducts; and it is thus clear that the epithelial lining of the entire system of bronchial subdivisions and ramifications is derived originally from the entodermal lining of the primitive foregut. By the close of the fourth month of fœtal life the columnar cells lining the trachea and bronchi have become ciliated.

At first the diverticula of the respiratory tube are surrounded by thick masses of mesoblastic tissue, but as development proceeds the latter fails to keep pace with the former, and hence the mesoblastic tissue becomes greatly reduced in amount and in thickness. Coincidently, this mesoblast becomes vascularised, and thus rich. plexuses of blood-vessels come to surround the terminal divisions of the epithelial tubes-an arrangement obviously adapted to the interchange of gases from air to blood and vice versa.

The rudiments of the developing lungs grow dorsally on each side of the œsophagus into the fissure-like portion of the cœlom which occupies the thoracic region. They push before them the endothelial lining of the cœlom, and thus come to acquire their covering of pulmonary pleura. By the development of the diaphragm and the pericardium the pleural portions of the cœlom become cut off from the peritoneal cavity and from each other.

THE DIGESTIVE SYSTEM.

REVISED AND LARGELY REWRITTEN

BY DAVID WATERSTON.

APPARATUS DIGESTORIUS.

The Digestive System.-The physical characters and the chemical composition of much of the food taken into the body are such that it cannot at once be utilised by the organism. Before it can be absorbed and used in nutrition it requires to be acted upon, both chemically and mechanically. The performance of these mechanical and chemical changes is known as digestion.

The term apparatus digestorius (digestive system) is applied collectively to the organs which are concerned in this process, in the reception of food into the body, and in the excretion of the undigested or unabsorbed residue.

The simple form of digestive system which is found in many of the lower animals consists of a simple tube, passing through the interior of the body, from an anterior or mouth aperture, to a posterior or anal orifice. The wall and lining membrane of the tube are so constructed as to act mechanically and chemically upon the food in its interior.

In man, a tube of this kind forms the basis of the digestive system. It extends from the mouth, through the neck, thorax, abdomen, and pelvis, to the anal orifice. But the tube, originally simple, has become modified, in different directions in different parts, for the performance of the various stages of the complex processes of digestion, absorption, and excretion.

The principal modifications which it has come to present are the following:(1) The tube is very greatly elongated, so that its total length measures from seven to eight times the length of the trunk. This is effected by the tube being thrown into folds or coils, especially in that part known as the small intestine.

(2) Certain portions of the wall of the tube have become modified in structure for the performance of special digestive changes. Thus, in the mouth there are found the teeth and tongue, for mastication or triturition of food and for deglutition, or swallowing. Further on in the course of the tube there is a dilated chamber, the stomach, in whose wall special glands, called gastric glands, are present, which produce the gastric juices; while in the succeeding portion, or small intestine, are found the villi-very numerous papillary projections of minute size, whose function is largely that of absorption.

(3) Certain special accumulations or masses of glandular tissue, producing secretions useful in digestion, are situated altogether outside of the wall of the tube, but communicating with its interior by means of ducts, through which these secretions are conveyed.

The chief of such masses of glandular tissue are the salivary glands, which are placed in the head and neck, and communicate with the mouth; and the liver and pancreas, which lie in the abdomen, and are connected with the duodenum.

These glandular masses, though lying external to the wall of the tube, have been developed as outgrowths from it, and the ducts represent the stalks of connexion.

The digestive system, then, may be considered to present the following parts:

I. The alimentary canal, or digestive tube.

II. Special organs, found in the wall of this canal.

III. Accessory glands, placed external to the wall of the tube.

FIG. 873.-DIAGRAM OF THE GENERAL ARRANGEMENT OF THE DIGESTIVE SYSTEM. The processus vermiformnis is seen hanging down from the cæcum. The transverse colon is not represented, in order that the duodenum and pancreas, which lie behind it, may be seen.

The greater part of the digestive system is found in the abdomen, and hence, in this section, the abdominal cavity, together with its lining membrane the peritoneum, falls to be described.

I. Alimentary Canal. The alimentary canal, taken as a whole, measures, when fully extended, about 30 feet (9 metres) in length, and consists (Fig. 873) of the

following parts in order :-mouth, pharynx, oesophagus, stomach, small and large intestines. The term tubus digestorius is applied to the whole of the canal below the lower end of the pharynx. The mouth cavity is the first division of the tube. It is separated from the nasal cavities above by the palate, and opens posteriorly into the pharynx. This latter is an expanded portion of the canal lying posterior to the mouth, nasal cavity and larynx, the mouth opening into it through the isthmus of the fauces, the nasal cavity through the choana (O.T. posterior nares); whilst lower down, immediately below the base of the tongue, the aperture of the larynx is found in its anterior wall. Opposite the lower border of the larynx, the pharynx is continued into the oesophagus, a long and comparatively straight portion of the digestive tube, passing through the neck and thorax to the abdomen, which it reaches by piercing the diaphragm. Immediately after entering the abdomen the tube expands into a pear-shaped dilated chamber, the stomach. This is followed by over 20 feet of small intestine, the junction of the two being marked by a constriction, the pylorus. The small intestine presents three more or less arbitrary divisions—namely, (a) the duodenum, a part about 10 inches in length, curved somewhat like a horse-shoe, and closely united to the posterior abdominal wall; (b) the jejunum, which includes the upper two-fifths, and (c) the ileum, the lower three-fifths of the small intestine beyond the duodenum. The jejunum and ileum are connected to the posterior abdominal wall by the mesentery, a fan-shaped fold of connective tissue covered by the peritoneum, or lining membrane of the abdominal cavity.

The terminal part of the ileum opens into the side of the large intestine, a few inches (2) from the blind commencement of the latter. There is thus formed at the beginning of the great intestine a cul-de-sac, called the cæcum, in connexion with which there is a small worm-shaped diverticulum, the vermiform

process.

The orifice through which the ileum opens into the large intestine is guarded by the valve of the colon (O.T. ileo-cæcal valve), which prevents the return of its contents from the large into the small bowel. After the cæcum comes the ascending colon, which runs up on the right side of the abdomen. This is succeeded, in order, by the transverse colon, crossing from right to left, the descending colon, running down on the left side of the abdomen, and the iliac colon, lying in the left iliac fossa. Beyond this are the pelvic colon, which lies in part or entirely within the pelvis minor (O.T. true pelvis), the rectum, and the anal canal.

The rectum lies within the pelvis minor, and the anal canal, the terminal part of the intestine, is a short channel passing between the muscles which form the pelvic floor, to open on the surface at the anal orifice.

The B.N.A. term colon sigmoideum includes the portion named above as pelvic colon, and the term colon descendens includes the descending and iliac colon.

II. Special Organs found in the Wall of the Tube.-In the mouth are found the teeth, gums, tongue, and behind them, in the pharynx, are the palatine tonsils. The teeth, 32 in number in the adult, are portions of the mucous membrane of the mouth and of the subjacent tissue, calcified on the surface, and specially formed for mastication, that is, the division and triturition of the food which take place in the mouth before the bolus, as the resulting mass is termed, can be swallowed. They are rooted in the jaws and are partly surrounded by the gums.

The tongue is a muscular organ, useful alike in mastication, deglutition, and speech. It is covered with epithelium, which in places is modified so as to form taste corpuscles, which are the end organs of the gustatory sense.

The roof of the mouth is formed by the palate, which separates the mouth from the nose. It consists of a bony part in front called the hard palate, and a movable sheet, called the soft palate, behind.

The palatine tonsils are two large masses of lymph tissue, found one on each side of the wall of the pharynx, just posterior to the mouth. They form the most prominent portions of an almost complete ring of lymph tissue placed around the circumference of the tube at this level.