McMurrich1914 Chapter 12
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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CHAPTER XII. THE DEVELOPMENT OF THE ORGANS OF RESPIRATION
The Development of the Lungs. - The first indication of the lungs and trachea is found in embryos of about 3.2 mm. in the form of a groove on the ventral surface of the oesophagus, at first extending almost the entire length of that portion of the digestive tract. As the oesophagus lengthens the lung groove remains connected with its upper portion (Fig. 182, A), and furrows which appear along the line of junction of the groove and the oesophagus gradually deepen and separate the two structures (Fig. 182, B). The separation takes place earliest at the lower end of the groove and thence extends upward, so that the groove is transformed into a cylindrical pouch lying ventral to the oesophagus and dorsal to the heart and opening with the oesophagus into the terminal portion of the pharynx.
Soon after the separation of the groove from the oesophagus its lower end becomes enlarged and bilobed, and since this lower end lies, with the oesophagus, in
the median attached portion of the dorsal edge of the septum transversum, the lobes, as they enlarge, project into the dorsal parietal recesses (Fig. 202), and so become enclosed within the peritoneal lining of the recesses which later become the pleural cavities.
The lobes, which represent the lungs, do not long remain simple,
33i
RP
Fig. 202. - Portion of a Section
THROUGH AN EMBRYO OF THE FOURTH
Week.
A, Aorta; DC, ductus Cuvieri; L, lung; O, oesophagus; RP, parietal recess; VOm, vitelline vein. - (Toldt.)
33 2
THE LUNGS
but bud-like processes arise from their cavities, three appearing in the right lobe and two in the left (Fig. 203, A), and as these increase in size and give rise to additional outgrowths, the structure of the lobes rapidly becomes complicated (Fig. 203, B and C).
The lower primary process on each side may be regarded as a prolongation of the bronchus, while the remaining process or processes represent lateral outgrowths from it. Considerable difference of opinion has existed as to the nature of the further branching of the bronchi, some authors regarding it as a succession of dichotomies, one branch of each of these placing itself so as to be in the line of the
/\
Vp \
V
c
Fig. 203. - Reconstruction of the Lung Outgrowths of Embryos of (/I) 4.3,
(5) 8.5, and (C) 10.5 MM.
Ap, Pulmonary artery; Ep, eparterial bronchus; Vp, pulmonary vein; 7, second lateral
bronchus; II, main bronchi. - (His.)
original main bronchus, while the other comes to resemble a lateral outgrowth, and other observers have held that the main bronchus has an uninterrupted growth, all other branches being lateral outgrowths from it, and the branching therefore a monopodial process. The recent thorough study by Flint of the development of the lung of the pig shows that, in that form at least, the branching is a monopodial one, and that from the main bronchus as it elongates four sets of secondary outgrowths develop, namely, a strong lateral, a dorsal, a ventral, and a weak and variable medial set.
THE LUNGS
333
There is a general tendency for the individual branches of the various sets to be arranged in regular succession and for their development to be symmetrical in the two lungs. But on account of the necessity under which the lungs are placed of adapting themselves to the neighboring structures and at the same time affording a respiratory surface as large as possible, an amount of asymmetry supervenes. Thus, it has already been noted that in the earliest branching a single lateral bronchus is formed in the left lung and two in the right. The uppermost of these latter, the first lateral bronchus, is unrepresented in the left lung, and is peculiar in that it lies behind the right pulmonary artery (Fig. 203, C), or in the adult, after the recession of the heart, above it, whence it is termed the eparterial bronchus. Its absence on the left side is perhaps due to its suppression to permit the normal recession of the aortic arch (Flint).
So, too, the inclination of the heart causes a suppression of the second ventral bronchus in the left lung, but at the same time it affords opportunity for an excessive development of the corresponding bronchus of the right lung, which pushes its way between the heart and the diaphragm and is known as the infra-cardiac bronchus.
As soon as the unpaired first lateral bronchus and the paired second lateral bronchi are formed mesenchyme begins to collect around each of them and also around the main bronchi, the lobes of the adult lung, three in the right lung and two in the left, being thus outlined. A development of mesenchyme also takes place around the excessively developed right second ventral bronchus, and sometimes produces a well-marked infra-cardiac lobe in the right lung.
Fig. 204. - Diagram of the Final Branches of the Mammalian Bronchi.
A, Atrium; B, bronchus; S, air-sac. - (Miller.)
334
THE LARYNX
In later stages the various bronchi of each lobe give rise to additional branches and these again to others, and the mesenchyme of each lobe grows in between the various branches. At first the amount of mesenchyme separating the branches is comparatively great, but as the branches continue, the growth of the mesenchyme fails to keep pace with it, so that in later stages the terminal enlargements are separated from one another by only very thin partitions of mesenchyme, in which the pulmonary vessels form a dense network. The final branching of each ultimate bronchus or bronchiole results in the formation at its extremity of from three to five enlargements, the atria (Fig. 204, A), from which arise a number of air-sacs (S) whose walls are pouched out into slight diverticula, the air-cells or alveoli. Such a combination of atria, air-sacs, and air-cells
constitutes a lobule, and each lung is composed of a large number of such units.
The greater part of the original pulmonary groove becomes converted into the trachea, and in the mesenchyme surrounding it the incomplete cartilaginous rings develop at about the eighth or ninth week. The cells of the epithelial lining of the trachea and bronchi remain columnar or cubical in form and become ciliated at about the fourth month, but those of the epithelium of the airsacs become greatly flattened and constitute an exceedingly thin layer of pavement epithelium. The Development of the Larynx. - The opening of the upper end of the pulmonary groove into the pharynx is situated at first just behind the fourth branchial furrow and is surrounded anteriorly and laterally by the PI -shaped ridge already described (p. 294) as
Fig. 205. - Reconstruction of the Opening into the Larynx in an Embryo of Twenty-eight Days, Seen from Behind and Above, the Dorsal Wall of the Pharynx being Cut Away.
co, Cornicular, and cu, cuneiform tubercle; Ep, epiglottis; T, unpaired portion of the tongue. - (Kallius.)
THE LARYNX 335
the furcula, this separating it from the posterior portion of the tongue (Fig. 178). The anterior portion of this ridge, which is apparently derived from the ventral portions of the third branchial arch, gradually increases in height and forms the epiglottis, while the lateral portions, which pass posteriorly into the margins of the pulmonary groove, form the ary epiglottic folds. When the pulmonary groove separates from the oesophagus, the opening of the trachea into the pharynx is somewhat slit-like and is bounded laterally by the aryepiglottic folds, whose margins present two elevations which may be termed the comicular and cuneiform tubercles (Fig. 205, co and cu, and Fig. 175). The opening is, however, for a time almost obliterated by a thickening of the epithelium covering the ridges,
Fig. 206. - Reconstruction of the Mesenchyme Condensations which Represent
the Hyoid and Thyreoid Carthages in an Embryo of Forty Days.
The darkly shaded areas represent centers of chondrification. c.ma, Greater cornu of
hyoid; c.mi, lesser cornu; Th, thyreoid cartilage. - (Kallius.)
and it is not until the tenth or eleventh week of development that it is re-established. Later than this, at the middle of the fourth month, a linear depression makes its appearance on the mesial surface of each ary-epiglottic fold, forming the beginning of the ventricle, and although at first the depression lies horizontally, its lateral edge later bends anteriorly, so that its surfaces look outward and inward. The lips which bound the opening of the ventricle into the laryngeal cavity give rise to the ventricular and vocal folds. The cartilages of the larynx can be distinguished during the seventh week as condensations of mesenchyme which are but indistinctly separated from one another. The thyreoid cartilage is represented at this stage by two lateral plates of mesenchyme,
336 THE LARYNX
separated from one another both ventrally and dorsally, and each of these plates undergoes chondrification from two separate centers (Fig. 206) . These, as they increase in size, unite together and send prolongations ventrally which meet in the mid-ventral line with the corresponding prolongations of the plates of the opposite side, so as to enclose an area of mesenchyme into which the chondrification only extends at a later period, and occasionally fails to so extend, producing what is termed a foramen thyreoideum.
The mesenchymal condensations which represent the cricoid and arytenoid cartilages are continuous, but each arytenoid has a distinct center of chondrification, while the cartilage of the cricoid appears as a single ring which is at first open dorsally and only later becomes complete. The epiglottis cartilage resembles the thyreoid in being formed by the fusion of two originally distinct cartilages, from each of which a portion separates to form the cuneiform cartilages (cartilages of Wrisberg) which produce the tubercles of the same name on the ary-epiglottic fold, while the corniculate cartilages (cartilages of Santorini) are formed by the separation of a small portion of cartilage from each arytenoid.
The formation of the thyreoid cartilage by the fusion of two pairs of lateral elements finds an explanation from the study of the comparative anatomy of the larynx. In the lowest group of the mammalia, the Monotremata, the four cartilages do not fuse together and are very evidently serially homologous with the cartilages which form the cornua of the hyoid. In other words, the thyreoid results from the fusion of the fourth and fifth branchial cartilages. The cricoid, in its development, presents such striking similarities to the cartilaginous rings of the trachea that it is probably to be regarded as the uppermost cartilage of that series, but the epiglottis seems to be a secondary chondrification in the glossolaryngeal fold (Schaffer). The arytenoids possibly represent an additional pair of branchial cartilages, such as occur in the lower vertebrates (Gegenbaur).
These last arches have undergone almost complete reduction in the mammalia, the cartilages being their only representatives, but,
LITERATURE 337
in addition to the cartilages, the fourth and fifth arches have also preserved a portion of their musculature, part of which becomes transformed into the muscles of the larynx. Since the nerve which corresponds to these arches is the vagus, the supply of the larynx is derived from that nerve, the superior laryngeal nerve probably corresponding to the fourth arch, while the inferior (recurrent) answers to the fifth.
The course of the recurrent nerve finds its explanation in the relation of the nerve to the fourth branchial artery. When the heart occupies its primary position ventral to the floor of the pharynx, the inferior laryngeal nerve passes transversely inward to the larynx beneath the fourth branchial artery. As the heart recedes the nerve is caught by the vessel and is carried back with it, the portion of the vagus between it and the superior laryngeal nerve elongating until the origins of the two laryngeal nerves are separated by the entire length of the neck. Hence it is that the right recurrent nerve bends upward behind the right subclavian artery, while the left curves beneath the arch of the aorta (see Fig. 149).
LITERATURE.
J. M. Flint: "The Development of the Lungs," Amer. Journ. Anal., vi, 1906.
J. E. Frazer: "The Development of the Larynx," Journ. Anat. and Phys., xliv, 1910.
E. Goppert: "Ueber die Herkunft der Wrisbergschen Knorpels," Morphol. Jahrbuch,
xxi, 1894. W. His: "Zur Bildungsgeschichte des Lungen beim menschlichen Embryo," Archiv
fiir Anat. und Physiol., Anat. Abth., 1887. E. Kallius: "Beitrage zur Entwickelungsgeschichte des Kehlkopfes," Anat. Hefle,
ix, 1897. E. Kallius: "Die Entwickelung des menschlichen Kehlkopfes," Verhandl. der Anat.
Gesellsch., xii, 1898. A. Lisser: "Studies on the Development of the Human Larynx," Amer. Journ.
Anat., xii, 191 1. A. Narath: "Der Bronchialbaum der Saugethiere und des Menschen," Bibliotheca
Medica, Abth. A, Heft 3, 1901. J. Schaffer: "Zur Histologie Histogenese und phylogenetischen Bedeutung der
Epiglottis," Anat. Hefte, xxxin, 1907. A. Sotjlie! and E. Bardier: "Recherches sur le developpement du larynx chez
l'homme," Journ. de V Anat. et de la Physiol., xxiii, 1907.
