Book - Quain's Embryology 8 - Revision history

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	==~~DEVELOPMENT OF THE LUNGS AND TRACHEA==~~
			=Development of Lungs and Trachea=

			:'''Development of Particular Organs and Systems'''

			{{Quain header}}


	The lungs first appear as two small protrusions upon the front of the oesophageal jiortion of the alimentary canal, completely hid by the rudimentary heart and liver. These primitive protrusions or tubercles are visible in the chick on the third day of incubation, and in the embryos of mammalia and of man at a corresponding stage of advancement. Their internal cavities communicate with the oesophagus, and are lined by a prolongation of the hypoblast. At a later period they are connected with the oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the bronchia and air-cells are developed by the progressive ramification of the internal cavity in the form of cffical tubes, after the manner of the ducts of glands.		The lungs first appear as two small protrusions upon the front of the oesophageal jiortion of the alimentary canal, completely hid by the rudimentary heart and liver. These primitive protrusions or tubercles are visible in the chick on the third day of incubation, and in the embryos of mammalia and of man at a corresponding stage of advancement. Their internal cavities communicate with the oesophagus, and are lined by a prolongation of the hypoblast. At a later period they are connected with the oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the bronchia and air-cells are developed by the progressive ramification of the internal cavity in the form of cffical tubes, after the manner of the ducts of glands.


	The diverticulum of hypoblast is surrounded by a mass of mesoblastic cells, so that the pulmonary parenchyma, like that of the glands, owes its origin to both hyiioblastic and mesoljlastic elements. The substance of the mesoblast, thickening round the primary diverticula, becomes penetrated by secondary diverticula formed from the hypoblast processes ; these are succeeded by tertiary branches which develop the bronchia, and ultimately have the air-cells formed as their terminations. The formative process consists essentially in the budding of hypoblastic into mesoblastic substance ; the hypoblast furnishing the lining epithelium of the tubes, and the mesoblast the other tissues, such as muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c.		The diverticulum of hypoblast is surrounded by a mass of mesoblastic cells, so that the pulmonary parenchyma, like that of the glands, owes its origin to both hyiioblastic and mesoljlastic elements. The substance of the mesoblast, thickening round the primary diverticula, becomes penetrated by secondary diverticula formed from the hypoblast processes ; these are succeeded by tertiary branches which develop the bronchia, and ultimately have the air-cells formed as their terminations. The formative process consists essentially in the budding of hypoblastic into mesoblastic substance ; the hypoblast furnishing the lining epithelium of the tubes, and the mesoblast the other tissues, such as muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c.


	In the formation of the trachea and bronchi the wall of the primitive oesophagus is projected downwards (or forwards), and by the gradual folding in of the sides a second median tube is separated from the primitive alimentary canal. This new tube grows out at its hinder end so as to bulge at the two sides, and thus the commencement of a right and left bronchus is formed. The subsequent division of the diverticular hollow goes on by budding of the hypoblast from within into the masses of pulmonary blastema. The division into larger lobes externally, three in the right and two in the left lung, may be seen at a very early period in the human foetus. As the bronchial subdivision extends within the lungs, a tubercular or coarsely granular appearance is seen over the outer surface, as observed by Kiilliker in the human foetus in the latter half of the second montli. This is produced by the primitive air-cells placed at the extremities of ramified tubes, which occupy the whole of the interior of the organ : the ramification of the bronchial twigs and multiplication of air-cells goes on increasing, and this to such an extent that the air-cells in the fifth month are only half the size of those which are found in the fourth month.		In the formation of the trachea and bronchi the wall of the primitive oesophagus is projected downwards (or forwards), and by the gradual folding in of the sides a second median tube is separated from the primitive alimentary canal. This new tube grows out at its hinder end so as to bulge at the two sides, and thus the commencement of a right and left bronchus is formed. The subsequent division of the diverticular hollow goes on by budding of the hypoblast from within into the masses of pulmonary blastema. The division into larger lobes externally, three in the right and two in the left lung, may be seen at a very early period in the human foetus. As the bronchial subdivision extends within the lungs, a tubercular or coarsely granular appearance is seen over the outer surface, as observed by Kiilliker in the human foetus in the latter half of the second montli. This is produced by the primitive air-cells placed at the extremities of ramified tubes, which occupy the whole of the interior of the organ : the ramification of the bronchial twigs and multiplication of air-cells goes on increasing, and this to such an extent that the air-cells in the fifth month are only half the size of those which are found in the fourth month.
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	Fig. 583. — Outlines of the anterior ~~HALF OF THE EjIBRYO ChICK~~ A'~~IEWED FROM BELOW~~, ~~SHOWING THE HeART IN ITS EARLIEST STAGES OF FORMATION~~ (~~after Kemak~~).		Fig. 583. — Outlines of the anterior half of the embryo chick viewed from below, showing the heart in its earliest stages of formation. (after Kemak).

			A, Embryo of abouut 20 to 30 lioxirs ; B, of about 36 to 40 hours ; a, anterior cerebi'al vesicle ; h, proto-vertebral segments ; c, amniotic fold ; 1, 1, primitive omphalo-mesenteric veins entering the Iieart posteriorly ; 2, their union in the auricle of the heart ; 3, the miLldle part of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.

	A, Embryo of abouut 20 to 30 lioxirs ; B, of about 36 to 40 hours ; a, anterior cerebi'al vesicle ; h, j)roto-vertebral segments ; c, amniotic fold ; 1, 1, primitive omphalo-mesenteric veins entering the Iieart posteriorly ; 2, their union in the auricle of the heart ; 3, the miLldle part of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.		{{Quain footer}}

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==DEVELOPMENT OF THE LUNGS AND TRACHEA==

The lungs first appear as two small protrusions upon the front of the oesophageal jiortion of the alimentary canal, completely hid by the rudimentary heart and liver. These primitive protrusions or tubercles are visible in the chick on the third day of incubation, and in the embryos of mammalia and of man at a corresponding stage of advancement. Their internal cavities communicate with the oesophagus, and are lined by a prolongation of the hypoblast. At a later period they are connected with the oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the bronchia and air-cells are developed by the progressive ramification of the internal cavity in the form of cffical tubes, after the manner of the ducts of glands.

The diverticulum of hypoblast is surrounded by a mass of mesoblastic cells, so that the pulmonary parenchyma, like that of the glands, owes its origin to both hyiioblastic and mesoljlastic elements. The substance of the mesoblast, thickening round the primary diverticula, becomes penetrated by secondary diverticula formed from the hypoblast processes ; these are succeeded by tertiary branches which develop the bronchia, and ultimately have the air-cells formed as their terminations. The formative process consists essentially in the budding of hypoblastic into mesoblastic substance ; the hypoblast furnishing the lining epithelium of the tubes, and the mesoblast the other tissues, such as muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c.

In the formation of the trachea and bronchi the wall of the primitive oesophagus is projected downwards (or forwards), and by the gradual folding in of the sides a second median tube is separated from the primitive alimentary canal. This new tube grows out at its hinder end so as to bulge at the two sides, and thus the commencement of a right and left bronchus is formed. The subsequent division of the diverticular hollow goes on by budding of the hypoblast from within into the masses of pulmonary blastema. The division into larger lobes externally, three in the right and two in the left lung, may be seen at a very early period in the human foetus. As the bronchial subdivision extends within the lungs, a tubercular or coarsely granular appearance is seen over the outer surface, as observed by Kiilliker in the human foetus in the latter half of the second montli. This is produced by the primitive air-cells placed at the extremities of ramified tubes, which occupy the whole of the interior of the organ : the ramification of the bronchial twigs and multiplication of air-cells goes on increasing, and this to such an extent that the air-cells in the fifth month are only half the size of those which are found in the fourth month.

Fig. 582. — Sketch iLLUSTRATiNa the DeveLOPMENT Oi THE RESPIRATORY ORGANS (from Fatlike). A. B O

A, oesophagus of a chick, on the fourth day of incubation, with the rudimentary lung of the left side, seen laterally ; 1, the front, and 2, the hack of the oesophagus ;

3, rudimentary lung protruding from that tube ; 4, stomach. B, the same seen in front, so as to show both lungs. C, tongue and resjjiratory organs of embryo of the horse; 1, tongue; 2, larjmx ; 3, trachea j

4, lungs seen from behind.

In birds the principal air-sacs, three in number, are formed in direct connection with the lung in the course of its early development, and the riidiments of these sacs may be seen at an early period, as bulging constituent parts of the rudimentary lungs.

===Pleurae===

Each lung receives a covering externally from the lining' membrane of the common pleuro-peritoneal cavity of its own side. This is at first only on the outer side; but, as the lungs enlarge, a fissure separates their solid substance from the outer wall of the oesophagus, and the pleura is carried round the lung-mass so as to encircle the gradually narrowing root of each lung. The two pleurse remain separated by the mediastinum and heart.

===Pulmonary Vessels===

The blood-vessels of the lungs which arise in the mesoblastic tissue seem to be of comparatively late formation.

penetrating into the mesoblast only on the twelfth day in the chick. The pulmonary arteries are developed in mammals in connection Avith the fifth branchial arch of the left side, but the manner in which they become connected with the vessels formed in the lung-substance, and the manner in which a union is established between the pulmonary veins and the left auricle have not yet been ascertained.

In the account of the general phenomena of development the establishment of the first circulation of blood, by the simultaneous formation of the simple heart and of the first blood-vessels and blood in the body of the embryo and in the vascular area of the blastoderm, has already been described, and in the General Anatomy (p. 180) an account has been given of the histological changes occurring in the first development of the blood-vessels and blood.

Fig. 583. — Outlines of the anterior HALF OF THE EjIBRYO ChICK A'IEWED FROM BELOW, SHOWING THE HeART IN ITS EARLIEST STAGES OF FORMATION (after Kemak).

A, Embryo of abouut 20 to 30 lioxirs ; B, of about 36 to 40 hours ; a, anterior cerebi'al vesicle ; h, j)roto-vertebral segments ; c, amniotic fold ; 1, 1, primitive omphalo-mesenteric veins entering the Iieart posteriorly ; 2, their union in the auricle of the heart ; 3, the miLldle part of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.