Human Embryology and Morphology 22

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter XXII. Respiratory System

Stages in the Evolution of the Human Respiratory System

The development of the lungs, the pleural cavities and chest waU forms one of the most complicated chapters of human embryology. The steps in the development of this system, as seen within the human embryo, are unintelligible until they are interpreted by a study of comparative anatomy, especially of those animal forms that show the manner in which a purely pulmonary system arose from one which was purely branchial. Hence it is necessary to briefly recapitulate the various modifications of the respiratory system which are seen to occur in ascending from the lowest to the highest class of vertebrates. Four stages may be recognized :

Stage I

This stage is represented in fishes, in which the respiratory system is made up of three parts : (1) Branchiae, in which the respiratory exchange of blood gases is efiected ; (2) the swim bladder, an evagination from the oesophagus, containing oxygen, and surrounded by lymphoid tissue ; (3) the musculature of the branchial arches and pharynx, which pumps water through the branchial clefts, and helps to force the blood through the branchiae ; (4) nerve system with centre — both motor and sensory — in the hind-brain, and visceral nerves supplied by the vagus, and from vasomotor centres in the dorsal region of the cord. Although branchiae are never developed in the human embryo, yet the condition in the 4th and 5th weeks, when the heart is subpharyngeal in position and the visceral and aortic arches are in process of development, can only be explained by the supposition that at one stage of evolution these parts had served a respiratory purpose.


Stage II

In most amphibians four parts are to be recognized in the respiratory system. (1) The swim bladder is bifid ; each half, now properly called a lung, projects within the abdominal cavity above the pericardium and liver (Fig. 362). (2) A respiratory passage leading from the pharynx to the lungs, and formed from the 2nd, 3rd, and 4th branchial (4th, 5th, and 6th visceral) arches. (3) The vascular system for each lung rises from the artery of the 6th visceral arch (Fig. 360). (4) The branchial muscles, which formerly forced water through the gill sHts, are now transmuted into pharyngeal muscles and help to pump air into the lungs — thus acting as muscles of inspiration. The muscles of the body wall (see Fig. 362) are modified to form the muscles of expiration. Two parts of these are specially worthy of notice, because in mammals they become the diaphragm : viz. (a) part of the transversalis sheet, which rises from the spine and ends in the pericardium, oesophagus and roots of the lung ; (6) a deep lamina of the rectus abdominis which ends in the pericardium. The nerve to these muscular segments descends on the outer aspect of the superior vena cava exactly in the same manner as the phrenic nerve descends to the diaphragm (Fig. 362).


600px Fig. 360. Showing the Pulmonary Artery arising from the 6th Aortic Arch in Human Embryo of 5 weeks. After Wilhelm His (1831-1904)

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Fig. 361. Showing that the Pulmonary Diverticulum arises between and behind the bases of the last or 6th pair of Visceral Arches. (Frazer.)

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Fig. 362. Diagram of the Lung and Respiratory Muscles of an Amphibian (Surinam toad) to show the Muscles out of which the Diaphragm is evolved. The lungs lie within the abdomen as in the 6th week embryo. The arrow, beginning over the apical region of the lung, shows the direction in which the mammalian lung develops. The shoulder girdle and greater part of the external oblique are cut away. The heart lies above the sternum.

Stage III

(1) In reptiles the lungs are abdominal in position, but an elaborate series of septa have grown up within them, thus exposing a larger vascular surface to the inspired air. (2) The respiratory passage is elongated and demarcated into larynx, trachea and bronchi. (3) Ribs and sternum are developed, so that the musculature of the body wall becomes differentiated into inspiratory and expiratory muscles.

Stage IV

In mammals an extraordinary developmental change occurs which leads to the formation of two pleural cavities and their complete separation from the abdomen by a diaphragm. The origin of the diaphragm must be sought- for, not in the reptiles, present or past, but in a very low form of amphibian. To understand the origin of the pleural cavities and diaphragm of mammals the following points must be kept in mind : (1) That the septum transversum, in its fully developed condition, as seen in the frog, is the fibrous layer of tissue which separates the heart from the liver, a corresponding structure is seen in the human embryo. (2) Into the septum transversum are inserted the deepest layer of the rectus abdominis and vertebral fibres of the transversalis (Fig. 362). (3) The ribs are developed in the intermediate layers of the body wall — between segments of the external and internal oblique muscles. The muscular fasciculi which end in the septum transversum are deep to the ribs and intercostal musculature. (4) The lung buds lie at first in the mesentery of the foregut from which they grow outwards on each side into a narrow (pleural) passage of the coelom, which leads from the pericardium to the peritoneal cavity (Fig. 363). The passage is situated at the upper border of the septum transversum ; its pericardial opening, the iter venosum, is closed by the superior vena cava. Now, when the lung buds grow out in the mammalian embryo, they fill these passa:ges and their hinder ends project into the abdominal cavity. Then in the 6th and 7th weeks the coelomic passage undergoes an extremely rapid expansion, growing into the body wall so as to separate the pericardium and the deeper or diaphragmatic layer of musculature from the outer or intercostal stratum. Lung growth follows closely on pleural expansion. The pleural cavities are in reality new chambers or spaces produced by an enormous expansion of the narrow coelomic or pleural passages of the embryo. We shall see that the septum transversum is also cleft during the expansion.



Fig. 363. Form of the Coelom in a Human Embryo of the 5th week. The arrow under the right duct of Cuvier is in the right passage leading from the pericardial to the peritoneal cavity. It is by the expansion of this coelomic passage that the right pleural cavity is formed.


The development of the diaphragm gave mammals two advantages : (1) an enormous increase in the power of inspiration ; (2) the respiratory negative pressure, which afiects all the viscera within the body cavity in reptiles, became restricted to the thorax in mammals.


Morphological Parts of the Respiratory System are : — (a) The respiratory passage which extends from the pharynx to the bronchioles of the lung. The tissues which surround this passage are derived from the coverings and substance of the 4th, 5th and especially the 6th arch. The nasal cavities continue the breath passages to the nostrils. We have seen how these cavities are shut ofi from the mouth in the later part of the 2nd month. (b) The pulmonary tissue made up of (1) a diverticulum from the fore-gut which represents the swim bladder ; (2) a vascular network derived from the capillaries of the fore-gut, into which opens a blood supply from the last (6th) pair of aortic arches (Fig. 360). (c) The respiratory muscles, sternum and ribs are formed in the somatopleure of the body wall.

Development of the Pulmonary System

In the 4th week, towards the end of it, a deep groove appears in the floor of the primitive pharynx and oesophagus. The groove or trough-like depression of the fore-gut commences between the ventral ends of the 6th (or 5th and 6th, see Fig. 361) arches and stretches almost to the stomach (Fig. 364). The furcula, formed from the central mass and ventral parts of the 4th segments, bounds the pulmonary groove in front (Fig. 319) ; in its anterior part, which is the most prominent, is developed the epiglottis ; the anterior parts of the lateral margins of the pulmonary groove, form the true vocal cords, for the arjrteno-epiglottidean folds are secondary formations of a later date (Frazer). The posterior parts of the margins of the groove unite, and in this manner the posterior part of the groove is separated as a diverticulum on the ventral aspect of the oesophagus (see p. 270). The anterior part of the groove represents the basis of the pulmonary passage ; the posterior part, the basis of the pulmonary tissue. Two points should be noted in connection with the relationships of the oesophagus at the 4th week : (1) like that of a fish, it is extremely short ; (2) it lies between the right and left cavities of the coelom in the dorsal attachment of the mesocardium of the sinus venosus (Fig. 367). (3) The part of the coelom which lies at each side of the oesophagus is the narrow passage connecting the pericardial and peritoneal cavities which becomes expanded to form the pleura.


Fig. 304. rioor of the Pharynx and Oesophagus of a Human Embryo of 4 weeks, showing the Furcula, Pulmonary Groove and Diverticulum. (After His.)


When the pulmonary outgrowth is viewed from the side, its posterior extremity is seen to end in a deep pocket, the pulmonary pocket or diverticulum (Figs. 276, 369). The wall of the pocket is lined by a mass of entoderm, 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. Round the pulmonary bud is grouped a mass of mesodermal tissue out of which the connective-tissue system of the trachea, bronchi and lungs is developed.


Fig. 365. The Trachea, Bronchi and Lung Buds in the 5th week of development. (After Broman.)

Fig. 366. The Lohulation of the Lungs early in the 6th week. (After Merkel.)



In the 5th week the pulmonary pocket produces a larger right and a smaller left process, the right and left lung buds (Fig. 365). The median j)art of the pulmonary outgrowth separates from the pharyngeal floor and forms the trachea. The anterior part forms the larynx (see p. 351). The right bud forms the right lung and bronchus ; the left, the left lung and bronchus. As the pleural cavities and their contained 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 6th arch. The rapid development of the lung during the 4th, 5th, and 6th weeks is illustrated by Figs. 276, 277, 365, 366. In the 4th week the lung bud is a mere diverticulum ;


Fig. 367. A Section of a Human Embryo to show the Relationships of the Pulmonary Buds at the 5th week, looking backwards. (After Kollmann.) in the 5th the trachea and buds of the main bronchi are apparent ; in the 6th week the secondary bronchi and separate lobes are in a process of differentiation.


In Fig. 367 the relationship of the lung buds is shown to surrounding structures during the 5th week. The following points should be noted : (1) As the lung buds grow out they push their way into the pleural passages — the narrow communications between the pericardium and peritoneum. These parts of the coelom form the pleurae. The part of the coelomic lining which is invaginated as a covering on the lung bud becomes the visceral pleura. The invaginating or ensheathing lining of the isthmus becomes the parietal pleura. 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, and completely separate the chest wall from the pericardium and diaphragm.


They meet after birth under the sternum, enclosing between them the anterior mediastinum.

(2) As will be seen from Fig. 363, the lung buds sprout out from the mesentery just behind the duct of Cuvier. This relationship is retained in the adult, the vena azygos major and superior vena cava lying above and in front of the root of the right lung. The roots of the lungs represent the situation at which the embryonic pulmonary outgrowth took place. If the left duct of Cuvier persisted it would lie above and in front of the root of the left lung. The ductus arteriosus — ^part of the 6th arch — lies over the root of the left lung. At this stage (5th week) the pleural passage or cavity is still in communication with both pericardial and peritoneal cavities. Its communication with the pericardium closes at the end of the 6th week.


Fig. 368. Transverse Section of a Human Embryo showing (1) the Outgrowth of the Lung Buds from the Mesentery of the Fore-gut ; (2) the Separation of the Pericardium from the Body Wall and Formation of the Pleural Cavities ; (3) the , Separation of the Diaphragmatic Lamina from the Septum Transversum. The arrow shows the direction in which the left pleura invades the body wall. (After Lockwood.)


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 (Fig. 365).


The condition of the lung buds during the 6th week is shown in Figs. 366, 370. Not only are the right and left bronchi formed, but so also 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 bronchioles and infundibula. Each bud is solid, and carries its slieath of mesoderm ; the appearance on microscopic examination is very similar to that of a gland, such as the pancreas or parotid. In the 3rd month the mesoderm between the pulmonary buds is extremely abundant ; by the sixth month it forms merely a thin stroma amongst the alveolar air sacs. At the sixth month saccular evaginations occur from the infundibula ; they form the air cells, or alveoli. Nothing is known definitely of the growth of the lung tissue after birth, but it is probably formed by outgrowths from the infundibula occupying the sub-pleural layer. The opinion usually held by embryologists is that the production of new alveoli ceases at the 7th month of foetal life. After that time there is merely an enlargement of the elements already formed.


1 R. Heiss, Anat. Anz. 1912, vol. 41, p. 62 (Dev. of Lobes of Lung).


Fig. 369. Diagram to show the manner in which the Heart is fixed within the Pericardium by the Arterial and Venous Mesocardia in a Human Embryo of 4 weeks. The " dorsal mesocardium " in the above figure forms part of the venous mesocardium.


Changes in the Shape of the Lung

Even in the 6th week the lungs are merely glandular masses round the terminal parts of the bronchial outgrowths. As in the frog, the hilum at this time forms the apex of the lung. During the 2nd and 3rd months the lungs assume their definite shape. The upper lobe grows towards the neck, and an apical region is thus formed. The diaphragmatic or basal surface is at first absent, but as the pleural cavities expand and the basis of the diaphragm is stripped from the body wall, this surface appears. In the human and anthropoid foetus the diaphragmatic or basal surface becomes remarkably large. The most important change, however, relates to the anterior or ventral border of the lungs ; at first situated on the dorsal side of the pericardium the lungs expand forwards until they reach almost to the lateral borders of the sternum. In man and anthropoids the ventral or sterno-costal part of the lung reaches a high degree of development.


Evolution of Air Sacs

In reptiles we see the original bladder-like lung becoming demarcated into two parts — an anterior or cephalic part with thick spongy walls which contain cellular recesses for air and are richly supplied with blood ; and a posterior, thin-walled and simple part. The thin-walled hinder part serves as a pulmonary bellows during the respiratory expansion and contraction of the body wall ; it naturally acts on the most yielding part of the lung. In birds, the anterior or respiratory part of the lung has been sharply demarcated from the posterior or " bellows " part. The latter is broken up into abdominal air sacs. In mammals the " bellows part," represented by the pulmonary infundibula and air sacs, is disseminated amongst the " respiratory " tissue and the bronchi are arranged in such a way as to permit every part of the lung to undergo expansion. Thus the pattern of the bronchial tree is determined by the pharynx larynx nature of the respiratory movements. Whereas only the respiratory part of a bird's lung is supradiaphragmatic the whole of the mammalian lung occupies this position.


Fig. 370. The condition of the Right and Left Pulmonary Buds in an Embryo at the end of the 6th week. (After His.)


There are certain peculiarities in the lungs of animals which are adapted to an upright posture (Man and Anthropoids) :

(1) Ramification of the Bronchi. — In quadrupedal mammals the main bronchus passes backwards in the lung as a main stem, which grows gradually smaller by giving off four dorsal and four ventral bronchial branches (Fig. 371). So altered are the human lungs, that the arrangement of bronchi seen in most mammals is not easily recognized in them. The ventral bronchi are larger, longer and more branched than in other mammals. In the human as in the mammalian lung the secondary and terminal bronchi are developed by a dichotomy or subdivision of the pulmonary buds.

(2) The Lobes of the Lungs. — In the embryonic condition (Fig. 370) 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 ; and this is also the conclusion wliicli Flint arrived at after a minute investigation of the development of the lungs of the pig. It must be remembered that the point of origin of any bronchus may easily be moved to meet new physiological conditions. At least in the human embryo each main bronchus gives ofi three primary buds. All three remain separate on the right side ; on the left the upper and middle primary buds arise together (Fig. 370). Hence the upper lobe of the left lung represents the upper and middle lobes of the right. In the sheep and pig the upper right lobe springs from the trachea. The bronchus of the upper right lobe (the reason for it is not clear) commonly lies above its artery — ^that is to say, it is eparterial. The other bronchi are hyparterial. A clue to the asymmetry of the right and left lungs will be found in a fuller knowledge of the mechanism of respiration.^

(3) The Diameters of the Thorax. — The peculiar branching of the bronchi in man and upright primates is due to the shape of the 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. 372). In upright animals (man, anthropoids, and also in 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.


Fig. 371. Scheme of the Bronchial Ramifications In Quadrupedal Mammals. B, the dorsal ramifications ; V, the ventral ramifications. Fig. 372. — Diagrammatic Section of the Thorax of a Quadrupedal Mammal {A), contrasted with a corresponding section in Man (£).


(4) 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 represents an over-development of the second ventral branch from the right bronchus (Fig. 366). It projects into and fills a slight recess between the pericardium and diaphragm, behind the intra-thoracic part of the inferior vena cava. This 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. 374). With the assumption of the upright posture (in man and anthropoids) the mechanism of respiration has become altered and the heart sinks until it rests on the diaphragm, the subpericardiac sinus and azygos lobe being thus obliterated. The reappearance of the azygos lobe as a separate structure — ^for a buried rudiment is always present — in man is an atavism — that is to say, a recurrence of an ancestral feature. In quadrupeds the contraction of the diaphragm is followed by an expansion of the lobus azygos and a corresponding elongation of the highly elastic intra-thoracic part of the inferior vena cava ; in man, on the other hand, the contraction of the diaphragm is followed by a descent of the heart, thus indirectly enlarging the pulmonary space.



^ Any one interested in this problem should consult Prof. Huntington's paper, Amer. Journ. Anat. 1920, vol. 27, p. 99.


Fig. 373. Showing the Origin of the Blood Supply to the Limg, in Cat Embryo. (Huntington.)


Blood Supply of the Lung

The pulmonary aorta is formed with the ascending part of the aortic arch, out of the truncus arteriosus (see p. 249). The right and left pulmonary arteries spring as branches from the right and left 6th aortic arches (Fig. 373). The lung buds are at first supplied by arteries arising from the dorsal aorta (Huntington), but in the 5th week this primary pulmonary plexus is joined by a communication from the Btli aortic arches, this anastomosis being the basis of the pulmonary arteries (Fig. 273). At first the pulmonary arteries descend by the side of the trachea, but as the heart becomes intra-thoracic in the 6th and 7th weeks they are gradually shortened until they pass horizontally to the roots of the lungs. The pulmonary veins grow out from the pulmonary buds and enter the left auricle through the venous mesocardium about the 5th week (Fig. 368). The mesenchymatous or interstitial tissue of the lungs is supplied by the bronchial arteries which represent the primary vessels of the lung buds (Fig. 373). These arteries also supply the pleura on the mediastinal and diaphragmatic surfaces of the lungs.


^ J. L. Bremer, Amer. Journ. Anat. 1901, vol. 1, p. 137 (Dev. of Pulmonary Arteries) ; V. Federow, Anat. Hefte, 1910, vol. 40, p. 529 (Dev. of Pulmonary Veins) ; Geo. S. Huntington, Anat. Bee. 1919, vol. 17, p. 165.


Changes at Birth

When the child begins to breathe at birth, the expansion of the lungs opens up the pulmonary circulation ; the foramen ovale is closed and the ductus arteriosus begins then to be closed, and within the 1st month becomes reduced to a fibrous cord. The ductus arteriosus represents the dorsal segment of the 6th left arch ; the corresponding part of the 6th right arch disappears soon after it is formed. It is not until about the 4th day after birth that the whole of the lung is inflated. The first part to expand is the costo-sternal or ventral part ; the second, the diaphragmatic or basal part, the apex is the third, and the dorsal border and deep part the last.^


Fig. 374. The Relationship of the Heart to the Diaphragm in Quadrupedal Mammals.


The Larynx

The larynx is developed round the anterior part of the pulmonary diverticulum. The origin of the cartilages of the larynx is shown in Fig. 375. The thyroid cartilage is formed by the expansion and amalgamation of the skeletal bases of the 4th and 5th visceral arches ; at least this is true of lower mammals, but in higher mammals only the 4th is involved (Edgewortli). the skeletal basis of the 6tli or pulmonary arch in man, which forms the two lateral cartilages in the short pulmonary passage of the frog, becomes divided into a dorsal segment which forms the arytenoid cartilage, a ventral segment to form the cricoid. From the posterior part of the primitive lateral cartilage arise the rings in the wall of the trachea, chief, secondary and ultimate bronchi (Fig. 375).


^ Eor papers relating to the morphology and mechanism of the lungs see Further Advances in Physiology, edited by Leonard Hill, 1909 ; also Keith, Journ. Anat. and Physiol. 1905, vol. 39, p. 243.

^ J. E. Frazer, Journ. Anat. and Physiol. 1910, vol. 44, p. 156 ; H. Lisser, Amer. Joxirn. Anat. 1911, vol. 12, p. 27 ; F. H. Edgeworth, Quart. Journ. Mic. Sc. 1916, vol. 61, p. 383.


Prof. Frazer has made a very thorough investigation of the development of the larynx. At each side of the primary pulmonary orifice lies a mass of tissue representing the last or 6th visceral arch (Fig. 361). In this tissue develops the various parts of the larynx. The cricoid and arytenoid are the primary cartilages ; they are the only ones present in the larynx of amphibia and reptiles. The thyroid only appears in mammals. The true vocal cords represent the primary opening of the larynx. In the 2nd and 3rd months of human development the part of the laryngeal cavity above the vocal cords (suprarimal part) is produced by the upgrowth of the lateral masses on each side of the primary opening. In these masses are developed the arytenoid cartilages and the aryteno-epiglottidean or permanent folds which bound the lateral margins of the secondary laryngeal orifice. The epiglottis, in Prof. Frazer's opinion, is developed out of the mass of tissue (central mass) which lies behind the 2nd and 3rd arches (Fig. 361).


Fig. 375. Diagram of the Cartilages of the Larynx to show the parts derived from the Skeleton of each Visceral Segment.


The muscles within the larynx are derived from the 6th visceral segment and are supplied by the inferior laryngeal nerve, while the crico-thyroid arises from the musculature of the 4th segment. In fish the pharyngeal orifice of the oesophagus is guarded and kept shut by a sphincter made up of striated muscle. When a pulmonary system is evolved the laryngeal or guarding musculature is derived from the primary sphincter of the oesophagus (Edgeworth).


The epiglottis is developed in the furcula (Symington) ; in lower vertebrates its lateral margins extend into the aryteno-epiglottic folds. The cartilages of Santorini and Wrisberg, in the aryteno-epiglottic folds, are continuous with the epiglottis in many mammals (Sutton). Until the 5th month of foetal life the epiglottis lies behind the palate and within the naso-pharynx — a position which is normal for the adults of many kinds of mammals.


The purposes which the larynx serves in all air-breathing vertebrates are (1) to regulate the inflow and outflow of respiratory air, and thus the positive and negative pressure within the lungs ; (2) to prevent food passing into the air passage. The production of voice which has led to a marked alteration of the human arytenoid cartilage is a secondary function. 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, although in a less specialized condition.^

Soon after the upgrowth of the lateral masses to form the suprarimal cavity of the larynx, an evagination takes place above each vocal cord to form the ventricles. In the 5th month mucous glands are developed from the membrane lining the ventricles, and a little later an outgrowth is developed from their apices to form the saccules of the larynx. They project against the thyro-hyoid membrane. Occasionally the saccule of the larynx may protrude through the thyro-hyoid membrane, thus giving rise to an air cyst in the neck. Laryngeal air-sacs are normally developed in anthropoids after birth, and attain a great size in the adults, extending to the chest and axillae. Their function is unknown.


Diaphragm

The diaphragm constitutes one of the most pronounced structural characteristics of mammals. The ancestral mammalian types in which the diaphragm first appeared are long since extinct ; we cannot study the evolution of the diaphragm among modern vertebrates. There are certain facts which throw light upon its origin, and make us certain that the diaphragm did not rise up gradually as a partition within the coelom and shut ofi that part which contains the lungs from the part containing the abdominal viscera. During the 4th and 5th weeks of development the pleural cavities are represented merely by the two short passages leading from the pericardial to the peritoneal cavity. In the 5th week the passages lie in the cervical region under the 4th and 5th spinal segments, from which the phrenic nerve arises, and from which the musculature of the diaphragm is derived. It is clear, then, that the diaphragm entered into the service of the lungs when these were situated, as in the frog, below the cervical region (Fig. 362). In some manner, as the lungs developed and afterwards took up a thoracic position, the muscle which became associated with them in the neck accompanied them when they retreated to their new position in the thorax. If we are to find a representative of the early form of the diaphragm, it must be amongst amphibians that we should look. We can also get light on its origin by studying certain malformations to which it is liable in man.


1 W. H. Duckworth, Journ. Anat. 1913, vol. 47, p. 82.

2 See Keith, Journ. Anat. and Physiol. 1905, vol. 39, p. 243 ; Mall, Bull. Johns Hopkins Hosp. 1901, vol. 12, Nos. 121-123, pp. 158, 171 ; I. Broman, Ergebnisse der Anat. 1911, vol. 20, p. 1 ; A. Brachet, Mem. de I'Acad. Boy. de Med. de Belgique, 1906, vol. 19 ; R. Mazilier, VEmbryologie du Diaphragme, Lille, 1907 ; Gladstone and Cockayne, Journ. Anxit. 1918, vol. 52, p. 64.


In Fig. 376 is shown the thoracic aspect of the diaphragm of a newly born child, in which the left pleuro-peritoneal opening has remained patent. Through the opening the upper end of the left supra-renal body and the spleen projected within the pleural cavity, giving rise to a congenital diaphragmatic hernia. The pleuro-peritoneal opening is situated on each side, between the muscular fibres which rise from the ribs and sternum, and which form the ventro-lateral part of the diaphragm, and the muscular fibres which arise from the spine and arcuate ligaments, forming the dorsal part of the diaphragm. The phrenic nerves, when they reach the diaphragm, divide into two branches, a ventral to the right and left ventrolateral parts (from 3rd and 4th cervical nerves), and a dorsal branch (from 4th and 5th cervical nerves) to the right and left dorsal parts. The central tendon, situated between the four parts just mentioned, makes up the fifth morphological element of the diaphragm. Each of these five parts — the central, the two dorsal and two ventro-lateral, has its own developmental history.


Fig. 376. The Thoracic Aspect of the Diaphragm of a newly born Child in which the communication between the Peritoneum and Pleura has not been closed on the left side ; the position of the opening is marked on the right side by the Spino-costal Hiatus. The dorsal mesentery of the fore-gut (represented by the posterior mediastinal pleura) is also shown.


The central tendon of the diaphragm is formed from the septum transversum (Fig. 377). The manner in which that structure is cleft into its pericardial and diaphragmatic elements by the outgrowth of the two pleural passages and lung buds has been already described (p. 342). The dorsal and ventral mesentery of the fore-gut (Fig. 379) are included in the formation of the septum transversum (p. 272), and hence the structures developed in these mesenteries — ^the aorta, oesophagus, azygos veins, thoracic duct, vagus nerves and inferior vena cava — perforate the median or central part of the diaphragm. The structures of the posterior mediastinum lie in the mesentery of the fore-gut (see Figs. 376, 378).


The ventro-lateral parts of the diaphragm are derived from the ventral longitudinal muscular sheets which give rise to the rectus abdominis and depressors of the hyoid bone (Fig. 362). Were the parts of this sheet restored to their embryonic relationships, then the pericardium should be placed beneath the mandible, so that the central tendon of the diaphragm lies opposite the 4th cervical segment. The sternal and costal origins of the ventro-lateral segment of the diaphragm should be detached in the thorax and the muscle placed ventrally in the neck so that it is continuous, at its insertion to the septum transversum, with the depressors of the hyoid bone. Behind the detached thoracic origins of the sternal and costal fibres should become continuous with the anterior end of the rectus sheet. In the human body the anterior part of the rectus sheet becomes divided into four strata — (1) the ventro-lateral fibres of the diaphragm, (2) the interchondral parts of the intercostals, (3) the rectus abdominis, which in all mammals, except man and the anthropoids, reaches forwards to the 1st rib, (4) the pectoralis major, minor, subclavius and that frequent human abnormality — the sternalis muscle. The development of the lung separates the deepest part of the rectus sheet from the chest wall to form the ventro-lateral part of the diaphragm. The ribs are formed in the chest wall and to the posterior six, this part of the diaphragm ultimately obtains an origin.


Fig. 377. A Lateral Section along the Thoracic and Abdominal Regions of a Human Embryo in the 5tla week of development, showing the Lung Bud gro^ring within the Septum Transversum and separating it into a Pericardial and a Diaphragmatic (costal) Lamina. Tlae arrow points to the dorsal mesentery of the fore-gut within which the crura of the diaphragm are developed. (After Mall.)


The dorsal parts of the diaphragm are formed from that part of the transversalis sheet of the body wall which forms the subvertebral musculature (Figs. 362, 376). The manner in which these parts of the diaphragm are detached from the body wall and carried into the thorax by the developing pleural cavities and lungs is shown in Figs. 377, 378. The right and left spinal parts of the diaphragm sink within the dorsal mesentery of the foregut, obtaining anteriorly an insertion to the pericardium and septum transversum, while posteriorly they retain an origin from the spine and costal processes. The quadratus lumborum, longus colli, the rectus capitis anticus major and minor are also derived from the subvertebral musculature.


Fig. 378. A Dorsal View of the hinder parts of the Expanding Pleural Cavities in a Human Foetus 16 mm. long and in the 7th week of development. (After Gladstone and Cockayne.) The pleuro-peritoneal openings are at the point of closure. Compare with Fig. 376. Arrows show the direction in which the Pleural Cavities expand into the Body Wall and separate the Pericardium from the Thoracic Parieties.


Pleuro-peritoneal Openings

The pleural passages, into which the lung buds develop at the end of the first month, open into the pericardium by the iterinera venosa ; behind they communicate with the peritoneum by the pleuro-peritoneal openings (Figs. 329, 330). These lie above the septum transversum (Fig. 379) and are separated by the mesentery of the fore-gut. In the mesentery between the openings are developed the spinal fibres of the diaphragm ; on the lateral side of each opening arise the costal fibres. The condition of the pleuro-peritoneal openings in the 7th week when they are on point of closing, is shown in Fig. 378. The actual closure is effected by that form of embryological healing to which the name of zygosis has been given (p. 287), but certain accessory factors are also involved in approximating their margins. (1) The spinal fibres migrate outwards and obtain attachment to the arcuate ligaments ; the costal fibres migrate inwards, obtaining an origin from the Uth and 12th ribs. Only in man and anthropoids does this migration occur, and the extent to which they approach each other and thus close the opening is extremely variable. (2) The collapsed condition of the lungs allows the abdominal viscera, developed in the domes of the diaphragm, to press the spinal and costal fibres against the dorsal wall of the thorax, thus mechanically closing the aperture. The liver, especially, by its upgrowth within the septum transversum helps to close the apertures, particularly on the right side, which is seldom the site of a diaphragmatic hernia. The supra-renal bodies are also developed just behind the pleuro-peritoneal orifices, and help to close them. Indeed, the mesentery of the Wolffian body, in the anterior extremity of which the supra-renal bodies develop, are attached along the dorsal wall of the coelom as far as the septum transversum, where it forms a fold upon the lateral or outer margin of the pleuro-peritoneal orifice. The developmental representative of this mesentery is sometimes named the pleuro-peritoneal membrane, and is regarded as an embryonic form of diaphragm.



Fig. 379. Section across Mesentery of the Fore-gut to show its relationship to the Pleuro-peritoneal Openings and Septum Transversum.


Musculature of the Body Wall

The development of the musculature of the body wall, also of the ribs and sternum, ought rightly to be included here, for all are closely related to the mechanism of respiration. The ribs have been already considered, and it will be more convenient to reserve the development of the wall of the thorax and abdomen with other correlated structures for another chapter (Chap. XXV.),





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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures