Book - Developmental Anatomy 1924-6

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Developmental Anatomy: Chapter I. - The Germ Cells and Fertilization | Chapter II. - Cleavage and the Origin of the Germ Layers | Chapter III. - Implantation and Fetal Membranes | Chapter IV. - Age, Body Form and Growth Changes | Chapter V. - The Digestive System | Chapter VI. - The Respiratory System | Chapter VII. - The Mesenteries and Coelom | Chapter VIII. - The Urogenital System | Chapter IX. - The Vascular System | Chapter X. - The Skeletal System | Chapter XI. - The Muscular System | Chapter XII. - The Integumentary System | Chapter XIII. - The Central Nervous System | Chapter XIV. - The Peripheral Nervous System | Chapter XV. - The Sense Organs | Chapter XVI. - The Study of Chick Embryos | Chapter XVII. - The Study of Pig Embryos | Figures
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter VI The Respiratory System

The development of the nose and pharynx are described elsewhere. Accordingly, the present account will deal exclusively with the origin of the other respiratory organs. In embryos of 23 segments, the anlage of this apparatus appears as a laryngo-tracheal groove in the floor of the entodermal tube, just caudal to the pharyngeal pouches. This groove produces an external ridge on the ventral wall of the tube which promptly becomes larger and rounded at its caudal end (Fig. 104). The groove and the ridge are the anlages of the larynx and trachea. The rounded end of the ridge is the unpaired anlage of the lungs; in embryos of 4 to 5 mm. it becomes bilobed.


Fig. 104. Stages in the early development of the trachea and lungs of human embryos (adapted by Prentiss). X about 50. A, 2.5 mm.; B, 4 mm.; C, stage B in side view; D, 5 mm. ; E, 7 mm.

Externally, two lateral longitudinal grooves mark off the dorsal esophagus from the ventral respiratory anlages. A fusion of the lateral furrows, progressing cephalad, constricts first the lung anlages and then the trachea from the esophagus. At the same time the laryngeal portion of the groove and ridge advances cranially until it lies between the fourth branchial arches (Fig. 87). At 5 mm., the respiratory apparatus consists of the laryngeal groove and ridge, the tubular trachea, and the two lung buds (Fig. 104 D).


The Larynx

In embryos of 5 to 6 mm., the oral end of the laryngeal groove is bounded on either side by two rounded prominences, the arytenoid swellings, which are continuous orally with a transverse ridge to form the furcula of His (Fig. 85). The transverse ridge becomes the epiglottis, derived from the third and fourth branchial arches (p. g6). In embryos of 15 mm., the arytenoid swellings are bent near the middle; their caudal portions lie parallel, while their cephalic segments diverge nearly at right angles (Fig. 105). The glottis, opening into the larynx, thus becomes T-shaped and ends blindly, as the laryngeal epithelium has fused. In fetuses of ten weeks this fusion is dissolved, the arytenoid swellings are withdrawn from contact with the epiglottis, and the entrance to the larynx becomes oval in form (Fig. 106). At eight weeks the ventndes of the larynx appear, and, during the tenth week, their margins indicate the position of the vocal cords. The elastic and muscle fibers of the cords are developed by the fifth month.



Fig. 105. The larynx of a 16 mm. human embryo (Kallius). X 15.

At the end of the sixth week, the cartilaginous skeleton of the larynx is indicated by surrounding condensations of mesenchyme, derived from the fourth and fifth pairs of branchial arches (p. 221). The cartilage of the epiglottis appears relatively late. The thyroid cartilage is formed from the fusion of two lateral plates, each of which has two centers of chondrification. The anlages of the cricoid and arytenoid cartilages are originally continuous; later, separate cartilage centers develop for the arytenoids. The cricoid is at first incomplete dorsad, but eventually forms a complete ring; it therefore may be regarded as a modified tracheal ring. The corniculatc cartilages represent separated portions of the arytenoids. The cuneiform cartilages are derived from the cartilage of the epiglottis. The laryngeal muscles originate in the fourth and fifth branchial arches and are consequently innervated by the vagus nerve which supplies those arches.

The Trachea

This tube gradually elongates during development, and its columnar epithelium becomes ciliated. Muscle fibers and the anlages of the cartilaginous rings appear in the condensed mesenchyme at the end of the seventh week. The glands develop as ingrowths of the epithelium during the last five months of fetal life.


The Lungs

Soon after the lung anlages, or stem buds, are formed (in 5 mm. embryos), the right bronchial bud becomes larger and is directed straighter caudad (Fig. 104). At 7 mm. the stem bronchi give rise to two bronchial buds on the right side, to one on the left. The smaller bronchial bud on the right side is the apical {eparterial) hud. The right and left chief buds, known as ventral bronchi, soon bifurcate. There are thus formed three bronchial rami on the right side and two on the left; these correspond to the primitive lobes of the lungs (Figs. 88 and 107). On the left side, an apical bud is interpreted as being derived from the first ventral bronchus (Fig. 107). It remains small so that there is no separate lobe corresponding to the upper lobe of the right lung. The absence of this upper left lobe may be an adaptation to permit the normal caudal regression of the aortic arch (p. 190).


Fig. 107. Ventral and dorsal views of the lungs from a human embryo of about 9 mm. (after Merkel). Ap., Apical bronchus; Di, D2, etc., dorsal, V i. V2, ventral bronchi; Jc., infracardiac bronchus.


The bronchial anlages continue to branch in srich a way that the stem bud is retained as the main bronchial stem (Fig. 107). That is, the branching is monopodial, not dichotomous, lateral buds being given off from the stem bud proximal to its growing tip. Only in the later stages of development has dichotomous branching of the bronchi and the formation of two equal buds been described. Such buds, formed dichotomously, do not remain of equal size (Flint). The inclination of the heart to the left suppresses one of the larger ventral bronchial rami on that side, but at the same time it affords opportunity for an excessive development of the corresponding right ramus which then projects into the space between the heart and diaphragm as the infrasardiac bronchus (Fig. 107, Jc).



Fig. 108. Transverse section through the lungs and pleural cavities of a 10 mm human embryo (Prentiss). X 23.

The entodermal anlages of the lungs and trachea are developed in a median mass of mesenchyme, dorsal and cranial to the peritoneal cavity. This tissue forms a broad mesentery, termed the mediastinum (Fig. 108). The right and left stem buds of the lungs grow out laterad, carrying with them folds of the mesoderm. The branching of the bronchial buds takes place within this tissue which is covered by the mesothelial lining of the future pleural cavity. The terminal branches of the bronchi are lined with entodermal cells; these flatten out and form the respiratory epithelium of the adult lungs. The surrounding mesenchyme differentiates into the muscle, connective tissue, and cartilage plates of the lung, tracheal, and bronchial walls. Into it grow blood vessels and nerve fibers. When the pleural cavities are separated from the pericardial and ])eritoneal cavities, the mesothelium covering the lungs, with the connective tissue underl3ung it, becomes the visceral pleura. The corresponding layers lining the thoracic wall form the parietal pleura. These layers are derived respectively from the visceral (splanchnic) and parietal (somatic) mesoderm of the embryo.


In 11 mm embryos, the two pulmonary arteries, from the sixth pair of aortic arches, course first lateral then dorsal to the stem bronchi (Fig. 109). d he right pulmonary artery passes ventral to the apical (eparterial) bronchus of the right lung. The single pulmonary vein receives two branches from each lung: a larger vein from each lower lobe, a smaller vein from each upper lobe, including the middle lobe of the right side. These four pulmonary branches course ventrad and drain into the pulmonary trunk. When this common stem is taken up into the wall of the left atrium, the four pulmonary veins open directly in to the latter.


According to Kolliker, the air cells, or alveoli, of the lungs begin to form in the sixth month and their developnent is completed during pregnancy. Elastic tissue appears during the fourth month in the largest bronchi. The abundant connective tissue found between the bronchial branches in early fetal life becomes reduced in its relative amount as the alveoli of the lungs are developed.


Until birth the lungs are relatively small, compact, and possess sharp margins. They lie in the dorsal portion of the pleural cavities. After birth the lungs normally fill with air, expanding and completely filling the pleural cavities. Their margins are then rounded and the compact, fetal lung tissue, which resembles that of a gland in strrrcture, becomes light and spongy, owing to the enormous increase in the size of the alveoli and blood vessels. Because of the greater amount of blood admitted to the lungs after birth, their weight is suddenly increased.


Anomalies

Variations occur in the size and number of lobes of the lungs; rarely there is a third lobe on the left side. The most common anomaly involving both esophagus and trachea is described on p. 104.


A striking malformation of the viscera in general is situs visccrum inversus, in which the various organs are transposed in position, right for left and left for right, as in a mirror image. This reversal may effect all the internal organs, or an independent transposition of the thoracic or abdominal viscera alone may occur. The early influence of the larger left great venous trunks is thought to be chiefly responsible for the usual positions and asymmetrical relations of the viscera.


Fig. 109. Ventral view of the lungs of a 10.5 mm. embryo, showing the pulmonary arteries and veins (His in McMurrich). X 27. Ep., Apical bronchus; I, u, primary bronchi.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Developmental Anatomy: Chapter I. - The Germ Cells and Fertilization | Chapter II. - Cleavage and the Origin of the Germ Layers | Chapter III. - Implantation and Fetal Membranes | Chapter IV. - Age, Body Form and Growth Changes | Chapter V. - The Digestive System | Chapter VI. - The Respiratory System | Chapter VII. - The Mesenteries and Coelom | Chapter VIII. - The Urogenital System | Chapter IX. - The Vascular System | Chapter X. - The Skeletal System | Chapter XI. - The Muscular System | Chapter XII. - The Integumentary System | Chapter XIII. - The Central Nervous System | Chapter XIV. - The Peripheral Nervous System | Chapter XV. - The Sense Organs | Chapter XVI. - The Study of Chick Embryos | Chapter XVII. - The Study of Pig Embryos | Figures

Reference

Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.


Cite this page: Hill, M.A. (2019, March 21) Embryology Book - Developmental Anatomy 1924-6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Developmental_Anatomy_1924-6

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