Book - Human Embryology and Morphology 17
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Chapter XVII. The Respiratory System
Introductory. — In the 3rd week the arrangement of - the heart and branchial arches in the human embryo is that of a water-breathing animal. In water-breathing animals the tubular heart expends its force in pumping the blood through the visceral arches — necessarily weakening the circulation in the rest of the body beyond them (Figs. 21 A and 21 B). The blood that returns to the heart and from which the heart is nourished is purely venous. In amphibians an airbreathing apparatus appears — hence the division in them of the auricular chamber, one chamber receiving systemic, the other pulmonary blood. In amphibians the air is pumped into the lung by the mouth and pharynx. In birds and mammals the division of the heart into pulmonary and systematic pumps becomes complete ; the chest wall is developed as a respiratory apparatus. The pharynx may be regarded as the respiratory organ of water breathers, and it is from the floor of this chamber that the pulmonary or air-breathing structures are developed. The arrangement of the heart and branchial arches in the human embryo can be explained only on the supposition that the airbreathing animals are the descendants of a water-breathing stock. The addition of the pulmonary system commences in the human embryo at the 3rd week.
Development of the Pulmonary System
In the 3rd week, towards the end of it, a deep groove appears in the floor of the primitive pharynx and oesophagus. The groove or troughlike depression of the fore-gut commences between the ventricle ends of the 4th arch and stretches almost to the stomach (Fig. 204). The furcula, formed from the ventral parts of the 4th arch, bounds the pulmonary groove ; in its anterior part, which is the most prominent, is developed the epiglottis ; its lateral margins, which bound the pulmonary groove, form the aryteno-epiglottic folds tuberculum impar. comes the upper aperture of the larynx. Two points should be noted in connection with the relationships of the oesophagus at the 3rd week: (1) like that of a fish, it is extremely short; (2) it lies between the right and left cavities of the coelom (Figs. 204 and 205) in the dorsal attachment of mesocardium of the sinus venosus (Fig. 202). (3) The part of the coelom which lies at each side of the oesophagus, is the narrow isthmus connecting the pericardial and peritoneal cavities which afterwards become the pleurae (Figs. 201 and 205).
Fig. 204. Floor of the Pharynx and Oesophagus of a human embryo of 3 weeks showing; the Furcula, Pulmonary Groove, and Diverticulum. (After His.)
When the pulmonary bud or groove is viewed from the side, its posterior extremity is seen to end in a deep pocket, the pulmonary pocket or diverticulum (Fig. 22, p. 30). The wall of the pocket is lined by a mass of hypoblast, which ultimately forms the epithelial lining of the whole respiratory tract, from the ciliated epithelium of the trachea to the pavement epithelium lining the alveoli of the lungs. Eound the pulmonary bud is grouped a mass of mesoblastic tissue out of which the connective-tissue system of the trachea, bronchi and lungs is developed.
In the 4th week the pulmonary pocket produces a larger right and a smaller left process, the right and left lung buds. The median part, developed from the hinder end of the groove, grows out from the pharyngeal floor and forms the trachea. The groove in front forms the larynx. The right bud forms the right lung and bronchus ; the left, the left lung and bronchus. The hypoblast becomes the epithelial lining of the respiratory tract ; the surrounding mesoblast forms the vessels, connectivetissue covering and coats of the respiratory tubes and lungs. As the lung buds develop the stomach is forced backwards ; the oesophagus becomes elongated. The tracheal part of the bud becomes separated from the oesophagus, but both retain the same nerve supply — the recurrent branch of the vagus — which is the nerve of the 5 th arch.
Fig. 205. A section of a human embryo to show the Relationships of the Pulmonary Buds at the 4th week, looking backwards. (After Kollmann.)
In Fig. 205 the relationship of the lung buds is shown to surrounding structures during the 4 th week. The following points should be noted : (1) As the lung bud grows out it pushes its way into the isthmus of the coelom — the narrow neck of communication between the pericardium and peritoneum (Fig. 201). This part of the coelom on each side forms the pleura. The part of the coelomic membrane which is invaginated on the lung bud becomes the visceral pleura. The invaginating or ensheathing lining of the isthmus becomes the parietal layer. As the lung buds grow, they distend the originally small pleural parts of the coelom until at the time of birth the right and left pleurae almost meet in front of the heart. They meet after birth under the sternum, enclosing between them the anterior mediastinum.
(2) As will be seen from Fig. 201, the lung bud sprouts out from the dorsal mesentery just behind the duct of Cuvier. This relationship is retained in the adult, the vena azygos and superior vena cava lying above and in front of the root of the right lung. If the left duct of Cuvier persisted it would lie above and in front of the root of the left lung. The lung bud springs from the pulmonary diverticulum just behind the 5th visceral arch of the pharynx. This arch is involved in the formation of the pulmonary groove and pulmonary diverticulum. It is from this arch (5 th aortic arch) that vessels arise, perforate the lung, and become the pulmonary arteries. The fifth arch — part of the branchial pharynx — takes a chief part in the formation of the pulmonary apparatus. The ductus arteriosus — part of the 5 th arch — lies over the root of the left lung. At this stage (4th week) the pleural cavity is still in communication with the peritoneal above the septum trans versum (Fig. 205).
Formation of the Bronchi and Lungs
The bronchi are the stalks of the right and left lung buds. The right bud is the bigger; the left is probably repressed by the heart turning to the left side. The right shows three secondary buds — the forerunners of the upper, middle and lower lobes of the lung; the left, two, which form the upper and lower lobes.
The condition of the lung buds at the end of the 5th week is shown in Fig. 206. The right and left bronchi are formed, so are the chief bronchial ramifications. Each ramification ends in a bud, which divides again and again and keeps on dividing until the fourth month. The terminal buds form the infundibula. Each bud is solid, and carries its sheath of mesoblast. At the sixth month sacular evaginations occur from the infundibula; they form the air cells, or alveoli.
Fig. 206. The condition of the Bight and Left Pulmonary Buds in a 5th week embryo. (After His).
There are certain peculiarities in the lungs of animals which are adapted to an upright posture (Man and Anthropoids) :
- Ramification of the Bronchi. — In quadrupedal mammals the bronchus does not divide by a process of equal dichotomy, as in man, but passes backwards in the lung as a main stem, which grows gradually smaller by giving off four dorsal and four ventral bronchial branches (Fig. 207). In man this arrangement can scarcely be recognised. The ventral branches in him have become larger than the main stems.
- The Lobes of the Lungs. — -In the embryonic condition (Fig. 206) it is seen that the right and left lung buds are nearly symmetrical. Aeby supposed the upper lobe of the right lung to be absent in the left, but that is not so. Each bronchus gives off three primary buds. All three remain separate on the right side ; on the left the upper and middle primary buds arise together (Fig. 206). Hence the upper lobe of the left lung represents the upper and middle lobes of the right. In the sheep the upper right lobe springs from the trachea. The bronchus of the upper right lobe (the reason for it is not clear) lies above its artery — that is to say, it is eparterial. The other bronchi are hyparterial.
- The Diameters of the Thorax. — The peculiar branching of the bronchi in man and upright primates is due to the shape of their lungs, which in turn is due to the shape of the thorax. In quadrupedal animals such as the horse or dog, in which the chest rests and is supported between the fore limbs, the thorax has its greatest diameter in the dorso-ventral direction (Fig. 208). In upright animals (man, anthropoids, and also in vertebra some water-living mammals, such as seals, etc.) the transverse diameter becomes the greater. At birth the diameters of the child's thorax are nearly equal. The thorax is flattened by the spine becoming invaginated within it ; the thorax thus comes to lie within the axis of gravity of the upright body.
- The Azygos Lobe. — On the inner side of the right lung of man the azygos lobe is frequently present, sometimes as a mere pulmonary projection or trace, sometimes as a lobule. It projects into and fills a slight recess between the pericardium and diaphragm, behind the intra-thoracic part of the inferior vena cava. The lobe is always well developed in quadrupedal mammals. In them the pericardium is separated from the diaphragm by a diverticulum of the right pleura — the sinus subpericardiacus (Fig. 209). With the assumption of the upright posture (in man and anthropoids) the heart sinks until its rests on the diaphragm, the sub-pericardiac sinus and azygos lobe being thus obliterated. The reappearance of the azygos lobe in man is an atavism — that is to say, a recurrence of an ancestral feature.
Fig. 207. Scheme of the Bronchial Ramifications in Quadrupedal Mammals. A, the Dorsal Ramifications ; B the Ventral Ramifications.
Fig. 208. Diagrammatic Section of the Thorax of a Quadrupedal Mammal (A), contrasted with a corresponding section in Man (B).
Fig. 209. The Relationship of the Heart to the Diaphragm in Quadrupedal Mammals.
Blood Supply of the Lung
The pulmonary aorta is formed, with the ascending part of the aortic arch, out of the bulbus or conus arteriosus (see page 230). The right and left pulmonary arteries spring as branches from the right and left 5 th aortic arches (Fig. 29, p. 37). They enter the lung buds, and are carried backwards with them. The pulmonary veins grow out from the pulmonary buds and enter the left auricle through the venous mesocardium about the 3rd month (see page 246).
Changes at Birth
When the child begins to breathe at birth the expansion of the lungs opens up the pulmonary circulation ; the foramen ovale starts to close and the ductus arteriosus begins then to contract, and within the 1st month becomes reduced to a fibrous cord. The ductus arteriosus represents the dorsal segment of the 5th left arch ; the corresponding part of the right 5th arch disappears soon after it is formed.
The larynx is developed in the floor of the pharynx out of the basal part of the pulmonary diverticulum. The origin of the upper aperture of the larynx has been already described (page 250). It is probable that the thyroid cartilage represents the skeletal parts of the 4th and 5th visceral arches, but this cannot be regarded as settled. There is no interval to be recognised between the hyoid bone and thyroid cartilage in the earlier stages of development. By some, the epiglottis is believed to be a derivative of the fifth arch.
Only in man and the higher anthropoids are the true vocal cords covered by stratified epithelium ; but all the muscles of the human larynx are represented in the larynx of the ape, but in a less specialized condition.
Occasionally the saccule of the larynx, a development from the apex of the ventricle, may protrude through the thyro-hyoid membrane, thus giving rise to an air cyst in the neck. Such laryngeal sacs are normally developed in anthropoids after birth, and attain in them great dimensions.
Fig. 210. Diagram of the Diaphragm to show the Parts formed by each of the five Elements.
The diaphragm is developed from five elements — one mesial from the primitive mesentery, two ventrolateral parts formed by adhesion, and two dorso-lateral formed much later (Fig. 210). In fishes, only the mesial or mesenteric element is developed ; in reptiles and birds the ventro-lateral elements are added, these three parts forming the septum transversum. In mammals the dorso-lateral elements are developed, and thus the pleural cavities become separated completely from the peritoneal. Occasionally in man the dorsolateral elements may fail ; a communication remains between the pleura and peritoneum, through which the abdominal contents may become herniated into the pleural cavity. This occurs eight times more frequently on the left than on the right side. The explanation will be found in the manner in which the liver is developed.
Elements entering into the Formation of the Diaphragm. 1. The Mesial or Mesenteric Element (Pig. 210). — When the splanchnopleures meet below so as to enclose the fore-gut, they form a mesial septum between the right and left halves of the coelom, with the gut suspended in it (Figs. 210 and 211). The part between the gut and the spine is the dorsal mesentery ; the part of the septum between the gut and the ventral wall is the ventral mesentery. In the ventral part of the mesentery below the fore-gut is swung the heart ; the mesocardia disappear ; but that part of the mesentery behind the heart, in which the sinus venosus rests and in which the stomach is developed, persists (Fig. 201.) The ventral and dorsal parts of the mesentery of this region form the first or mesial element of the diaphragm. In this part of the mesentery lie (Figs. 211 and 212) (1) the stomach, (2) the posterior end of the sinus venosus ; (3) the vitelline and umbilical veins which pass in it to the sinus; (4) the ducts of Cuvier which enter it to reach the sinus ; (5) the liver bud which is developed into it ; (6) the ductus venosus and pre-renal part of the inferior vena cava which are developed in it. Further, on each side of this part of the mesentery lies the isthmus of the coelom into which the lung buds are developed and out of which the pleural cavities are formed (Fig. 205). The mesial element is situated, at first, under the middle cervical segments of the trunk.
Fig. 211. Diagrammatic section behind the Embryonic Heart to show the Part of the primitive Mesentery which forms the mesial Element of the Diaphragm.
In the course of development the stomach gradually separates itself from this part of the diaphragm ; the liver bud also frees itself ; the ducts of Cuvier, with the sinus venosus, also become separated from it.
2. The formation of the ventro-lateral parts of the diaphragm is obscure. They appear very early (3rd week), while yet the vitelline veins terminate in the sinus venosus, and before the liver bud has grown out (Fig. 187, p. 229). It will be seen from Figs. 187 and 212 that the vitelline veins pass obliquely in the ventral mesentery from the yolk sac to the sinus venosus. They are said to be the active agents in producing the ventro-lateral parts of the diaphragm. They become so dilated (see Fig. 211) as to produce their covering of splanchnopleure outwards until lateral folds are formed which come in contact with, and adhere to, the somatopleure. In this way the mesentery (mesial part of the diaphragm) becomes adherent to the lateral somatic walls. These three parts, mesial and ventro-lateral, form the septum transversum.
3. The Dorso-lateral Parts. — These parts are formed in the 3rd month, and arise as cresentic folds from the dorsum of the isthmus. They are situated at first under the cervical region and their free cresentic edges are directed backwards over the lung buds. They are developed in mammals only, and are evidently comparatively late structural additions. In the - formation of these parts of the diaphragm, which close up the pleuro-peritoneal communications, probably several other factors take part. Of these may be mentioned : (1) The Wolffian ridges, which extend forwards as far as the cervical region, and project from the roof of the coelom at the passage between the peritoneum and pleura.
(2) The formation of the supra-renal body (Minot). It is developed in the mesoblast round this opening — -partly in the septum, partly in the Wolffian ridge.
(3) The growth of the liver within the septum transversum probably plays an important part. Hence, probably, the greater frequency of diaphragmatic hernia on the left side, where the liver growth is less. By these various factors the openings are closed and the diaphragm completed by the formation of the region of the arcuate ligaments.
The diaphragm begins to develop in the neck under the 4th and 5th cervical segments. Processes from the muscle plates of these two segments enter the basis of the diaphragm and form part at least of its muscular substance. The muscle buds carry their nerve — the phrenic — with them. The descent of the heart, the retrogression of the stomach, and the development of the lungs lead to the diaphragm being pushed backwards until it assumes the adult position.
Development of the Supra-renal Bodies
In spite of much research there is still doubt as to the origin and nature of these bodies. The cortex and medulla are certainly of different origin. In Elasmobranchs (sharks, etc.) these two elements are separate, the cortical part forming an inter-renal, unpaired body; the medullary part a body situated above the Wolffian kidney (suprarenal) on each side, and closely connected with a ganglion of the sympathetic system, in connection with which they are developed. In vertebrates above fishes the cortical and medullary parts are combined in one body. The medulla arises from the groups of cells which form the sympathetic ganglia; probably from the primitive cell basis of the semilunar ganglion, which is developed in the septum transversum, close to the pleuro-peritoneal canal. Hence the great plexus of nerves which pass from the solar plexus to the medulla of the supra-renals. By the 3rd month in the human foetus these cells have lost all evidence of their origin from nerve cells, and have taken on a chromogenic function. Probably the intercarotid and coccygeal bodies are similar to the medulla in origin and nature (Swale Vincent).
The cortical part is of quite different origin. It is developed within the Wolffian ridge and causes or assists to cause, the closure of the pleuro-peritoneal canal over which it is formed. It arises from the endothelium covering the Wolffian ridge — that part of the ridge which lies above the diaphragm in the pleuroperitoneal canal. The endothelium proliferates within the Wolffian ridge and comes in contact with the medullary element derived from the sympathetic. The medullary element is at first the larger and surrounds the cortical ; but soon the increased growth of the cortex leads to its surrounding and containing the medulla. The cortical cells range themselves in rows between blood sinuses. Probably cortical cells invade and replace the medullary part (Minot). As the kidneys grow forwards they come in contact with the supra-renal bodies which at first lie on their ventral surface. The supra-renal is at first larger than the kidney ; even at birth they are nearly equal in size. The nerves and arteries enter the bodies on their renal surface ; the veins emerge on their anterior surface.
Isolated parts of the supra-renal body (accessory supra-renals) occasionally occur in the broad ligament or in the spermatic cord above the testicle. Such accessory bodies are probably derived from the cortical element which is developed within the Wolffian ridge and body. With the descent of the ovary and testicle, which bring with them the Wolffian body, adjacent accessory supra-renals, if such be present, are also brought down, and may occasionally give rise to peculiar tumours.
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
Cite this page: Hill, M.A. (2020, November 29) Embryology Book - Human Embryology and Morphology 17. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_and_Morphology_17
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