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the wall. As the tubes diminish, the cartilaginous deposits show a corresponding reduction in size, until at last, in bronchi of 1 mm. diameter, they disappear altogether. The glands in relation to the tubes for the most part cease to exist about the same point. The muscular or middle coat, which in the trachea and primary bronchi is confined to the dorsal wall of the tube, forms a continuous layer of circularly arranged bundles in the bronchi as they ramify within the lung. Spasmodic contraction of the muscular coat gives rise to the serious symptoms which accompany asthmatic affections. The muscular fibres of the middle coat may be traced as far as the atria, on the walls of which they are present in considerable numbers. The mucous lining of the tubes becomes greatly thinned as it is followed into the smaller bronchioles. It contains a large number of longitudinally arranged elastic fibres, and is disposed in longitudinal folds, so that when the tube is cut across the lumen presents a stellate appearance. The mucous membrane is lined with ciliated columnar epithelium.
Structure of the Atria and Alveoli.-The walls of the atria and alveoli are exceedingly fine and delicate, but, nevertheless, constituents continuous with those observed in the three coats of a bronchus are found entering into their construction. The epithelium is reduced to a single layer. Further, it is no longer columnar and - ciliated, but it has become flat and pavement-like. Two kinds of epithelial cells may be recognised (1) a few small granular polygonal cells, arranged singly or in groups of two or three, (2) more numerous thin cells of large size and somewhat irregular in outline. Outside the epithelium is a delicate layer of faintly fibrillated connective tissue. This is strengthened by a network of elastic fibres, which is specially well marked around the mouths of the alveoli, and is also to some extent carried over the walls of the air-cells. Muscular fibres also are present on the walls of the atria, but it is questionable if any are prolonged over the air-cells.
Pulmonary Vessels.-The pulmonary artery, as it traverses the lung, divides with the bronchi, and closely accompanies these tubes. The resultant branches do not anastomose, and for the most part they lie above and dorsal to the corresponding bronchi. The fine terminal divisions of the artery join a dense capillary plexus which is spread over the alveoli or air-cells. This vascular network is so close that the meshes are barely wider than the capillaries which form them. In the partition between adjacent alveoli there is only one layer of the capillary network, and thus the blood flowing through these vessels is exposed on both aspects to the action of the air in the air-cells. The radicles of the pulmonary vein arise in, and carry the blood from, the pulmonary capillary plexus. Each afferent arteriole supplies the blood which flows through the capillaries spread over a number of neighbouring alveoli, and in like manner each afferent venous radicle drains an area corresponding to several adjoining air-cells. At first the veins run apart from the arteries, but after they have attained a certain size they join them and the bronchi. As a rule the pulmonary veins are placed on the inferior and ventral aspects of the corresponding bronchi.
DEVELOPMENT OF THE RESPIRATORY APPARATUS.
The larynx, trachea, bronchi, and lungs all arise as an outgrowth from the ventral aspect of the foregut. The first indication of a respiratory tract occurs in the human embryo early in the third week, on or about the fifteenth day of development, and when the embryo is but little more than 3 mm. in length. At that period a median longitudinal groove makes its appearance in the ventral wall of the foregut, extending from the primitive pharynx well towards the primitive stomach, and deepening gradually as it passes caudalwards.
The cranial end of the respiratory tube becomes enlarged and forms the larynx, the intermediate portion forms the trachea, and the caudal end bifurcates in the floor of the groove into two tubes-the future bronchi are already indicated by slight bulgings before the two tubes divide-which grow caudalwards on either side of the heart, into a mesodermic mass, from which the connective tissue of the future lungs is ultimately developed. The respiratory tube is lined with entoderm continuous with the entodermal lining of the foregut.
The groove becomes deeper and constricted, its lateral margins approximate, and finally meet dorsally, and the groove separates off from the foregut as a distinct tube. This differentiation necessarily results in the production of two tubes or canals, a ventral one forming the respiratory tube, and a dorsal one the oesophagus. The separation of the two tubes commences at the caudal end and proceeds cranialwards towards the pharynx, into which both the œsophagus and the respiratory tube open.
The Larynx. The rudiment of the larynx appears, at the cranial or pharyngeal end of the primitive respiratory tube, about the twenty-fifth day, and before the trachea separates off from the oesophagus in the form of two lateral swellings-the arytenoid swellings, which lie caudal to the fourth visceral pouches, and possibly represent rudimentary fifth branchial arches (Kallius). The arytenoid swellings are connected by a ventral median ridge which intervenes between the ventral ends of the third visceral arches. At this period the site of the future larynx is represented at the pharyngeal end of the respiratory tube by a U-shaped ridge which surrounds the tube cranially and laterally, and is known as the furcula.
The cranial or anterior portion of the furcula forms a median elevation from which the epiglottis is developed, whilst the lateral portions of the furcula-the arytenoid swellings eventually form the ary-epiglottic folds. On the medial side of the latter, about the fourth month, a furrow marks the future site of the ventriculus laryngis [Morgagni], the margins of which later become the vocal folds.
About the eighth week the cartilaginous framework of the larynx is indicated by mesoblastic condensations of the connective tissue around the now slit-like rima glottidis: and at the same period the rudimentary arytenoids, the cricoid and the cartilages of the trachea are all continuous laterally.
The epiglottic cartilage is developed, as stated, in the anterior portion of the furcula, and chondrifies relatively late. It may possibly represent a rudiment of the cartilage of the sixth branchial arch, and according to Göppert it is at first continuous dorsally with the cuneiform cartilages, which, therefore, are derivatives of the epiglottic cartilage.
The thyreoid cartilage is laid down in the form of two separate lateral mesoblastic plates, in each of which chondrification proceeds from two centres, ventral and dorsal, which probably represent the cartilages of the fourth and fifth branchial arches. development proceeds the sheets of cartilage formed from these centres fuse, and eventually extend ventrally to fuse with their fellows of the opposite side, in the median plane. Chondrification is completed comparatively late, and when incomplete it results in the formation of an abnormality-the thyreoid foramen. The superior cornu of the thyreoid cartilage is at first continuous with the greater cornu of the os hyoideum, and the remains of this cartilaginous connexion is seen in the presence of the cartilago triticea in the lateral hyothyreoid ligament of the adult.
The pro-cartilaginous rudiments of the cricoid and arytenoid cartilages are at first continuous with each other, but later become differentiated by the appearance of separate cartilaginous centres for the arytenoids, and an incomplete ring, for a time deficient dorsally, for the cricoid. The cricoid thus resembles developmentally a tracheal ring, with which it probably corresponds morphologically. Chondrification proceeds in the cricoid by two centres, one on each lateral side. These centres unite ventrally, but dorsally fusion does not take place until much later, and is finally completed by an extension of chondrification from the lateral into the dorsal plate. The cricoid thus differs from the tracheal ring, in having its chondrification completed dorsally, whereas this never takes place in the tracheal ring.
The arytenoid cartilages are, as stated, at first continuous with the cricoid cartilage by fibrous tissue, but become eventually completely separated from it by the appearance of one chondrification centre for each arytenoid.
The corniculate cartilages (Santorini) are merely portions of the arytenoid cartilages separated off by segmentation; whilst the cuneiform cartilages (Wrisbergi) are, as previously stated, derivatives of the epiglottic cartilage.
The Trachea. The trachea is developed from the intermediate portion of the median longitudinal groove. Originally, both this portion of the primitive respiratory tube and the œsophageal portion of the primitive alimentary canal were of equal length. but as development proceeds both tubes lengthen, the latter more rapidly than the former, so that eventually the lung rudiments no longer lie on the ventral and lateral sides of the primitive stomach, but come to lie on the cephalic side of that viscus and are separated from each other by the oesophagus dorsally and the heart and pericardium ventrally. In this way, that is by unequal growth, it comes about that the trachea in the adult is shorter than the oesophagus, though originally both were of equal length.
The cartilaginous rings of the trachea are developed like the cricoid cartilage, with the difference that in the trachea the process of chondrification does not extend into their dorsal portions, and hence, in the adult, the C-shaped rings of the trachea are deficient dorsallyan arrangement which admirably adapts itself to the functional uses of both trachea ani œsophagus..
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 I 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 foetal 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 coelom, 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.
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.