Paper - On the origin of the pulmonary arteries in mammals
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Bremer JL. On the origin of the pulmonary arteries in mammals. (1902) Amer. J Anat. 1(2): 135-
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I. On the Origin of the Pulmonary Arteries in Mammals
From the Embryological Laboratories of Harvard Medical School, Boston, Mass.
With 9 Text Figures.
The material used in preparing this paper is from the collection of the Laboratory of Embryology at the Harvard Medical School; the original numbers of the series and Sections have been preserved. The drawings are from reconstructions, and represent, as it were, casts of the lumina of the arteries without referenoe to the thickness of their walls. They are all of the same magnification ()( 80 diameters); the arteries are seen from behind, and the pulmonary arches can be followed until they unite to form the truncus pulmonalis, or until, as in Fig. 1, they enter the heart itself.
1n 1857, I-I. Rathke published his monograph, «D«ic Aortcnwässzcln und die von, Ohne« easy-abends«- Artersien des« Bankiers« in which appear the diagrams of the aortic arches now made more familiar by their repro duction by Kölliker, Hertwig, Quain, and many others, with or without slight modificationa In these diagrams the right and left pulmonary arteries are represented as arising, in lizasrds and birds, from their respective fifth, or pulmonary, arches, while in snakes and in mammals one fifth arch alone gives rise to both pulmonary arteries, the other arch becoming obliteratedz in snakes the right pulmonetry arch remains, in mammals the left. since this monograph there has been, so far as I know, no special investigation into the origin of the pulmonary arteries.
The ear1iest buds of the pulmonary arteries, in the rabbit, appear in embryos of about 4.0 mm., one bud from each of the psnlmonary arches, on the mesial aspect of each. The growth of these buds is at first backward, then downward and inward, giving a small twist, Figs. It, L, Z, X, near the proximal end of the pulmonary artery, which seems peculiar to the ra-bbit. From this weist, the course is straight downward, on each side of the trachea and slightly anterior t.o it, to the lungs, where the usua.l branches are given ofk. During this downward course no branches are seen. As the arteries increase in length their proximal ends, where they arise from the aortic arches, seem to approach each other actually, as can be seen by comparing Figs. 1, 2 and Z. The mechanism of this change is prosbably as follows: the truncns pulmonalis is at first short, soon dividing into its two manches, the right and left jifth aortic archesz as it becomes twisted around the aorta, following the turn of the heart, the trnncus pulmonalis pulls on the two fifth arches, which are thus crowded together, forming a, double tube, and at the same time the two pulmonary arteries, arising from the mesial aspect of the two arches, are brought nearer to-gether. By fnsion of the two parallel arches the trnncus pulmonalis is increased in length, and its-two branches shortss enedz this fusion may extend until the origins of the pnlmonaisy arteries are very near the bifurcation, or until the left artery springe actually from the bifurcation.
Fig. 1. Rabbit of 5.0 mm. Frontal series No. 148, Sections 250-261. )( 80 diameter-s. A, B, left and right pnlmonary arches, opening directly into heart, H. c, D, pulmonkiry arteries. F, G, fourth aortic arches, opening into Idee-Ist.
Fig. 2. Rabbit of. 8.0 mm. Frontal series No. 154, Sections 291—311. )( 80 diameter-s. B, junction of A and B. H, valve of heart.
The diameter of the pulmonary art-eries remains small in comparison to their increasing length, as one might expeot from the slight necessity of blood in the unused lungs. The left pulmonary arch grows rapidly in diameter as well as in length, while the right becomes entirely obliterated beyond the point vvhere the pulmonary artery arises, leaving finally no trace of its existenoez from this point to the junction with the left arch to form the truncus pu1monalis, the right arch remains of the same oalibre as the pulmonary artery. The small twist inarking the origin of the pulmonary artery gradually straightens out, and the whole right side, es. e. the anterior portion of the fifth arch and the pulmonaxry artery, being now unattached to the right dorsal aorta, is drawn to the left by the larger left aortic arch, which is constantly tending to become straight As a result of these changes, the left pulmonary areh seems to give rise, at about its mid-point, to two arteries, with their origins close together (or there may be a very short common stem); the right one, the longer of the two, arising anteriorly, and talcing its course at first almost horizontally across to the right side of the trachea, then bending down toward the right lang, the left pursuing a straight course to the left lung. The portion of the left jifth arch posterior to the pulmonary arteries becoimes later the Ductus Botalli, and is closed at birth.
Fig. 3. Rabbit of 10.0 mm. Frontal series No. 157, Sections 347-367. X 80 diameters.
Fig. 4. Pig of 7.8 mm. Frontal series No. 430, Sections 270-297. X 80 diameters
It will be seen from this description that, in the actual origin of the pulmonary arteries, the rabbit is identical with birds and repti1es, as drawn by Rathke and verified by many other writers. In the rabbit, as well a-s in birds and reptiles, one pulmonary artery arises from each pulmonary arch, but in birds and reptiles the growth of these arches is equal until birth, so that the picture is symmetrica1, a fifth arch, a pulmonary artery, and a Ductus Botalli on each sidez while in the rabbit the left pulmonary arch alone remains until birth, and the pieture is distorted It was this distortion, this early disappearance of that portion of the right pulmonary arch posterior to the pulmonary artery, which made possib1e the diagram of Rathke, and his statement that in mammals the left fifth aortic arch at a very early period of embryonic life sends out from about its mid—point a small branch which is intended for both lungs, and posterior to its place of origin divides into two twigs.
Bathlce examined, of mammals, the pig, sheep, and hare, with special reference to the pulmonary arteries. In the rabbit, cat, and in the few human embryos within my reach, I have found the pulmonary arteries to arise as I have stated, that is, in the beginning, symmetrica11y, one from each pulmonary arch. In Rathke’s original diagrains the arteries of lizards and of birds arise symmetrica.lly, as do they also in the frog, as described by Gauppsz Of snakes, according to stannius and others, while most species have only the right lung, and therefore only the right pulmonary artery, in adult life, some species have the left lung and left artery alone, and others even both lungs and both arteries, more or less fully developed In two cases, recently cited by F. Hoch— Stett-ers of Tropidonotus tessellatus (a) specie-s vxsith only the right lung normally developed), a slender artery was found, which, although iinally ramifying in the oesophageal Wall, resemb1ed in origin and course a left pulmonary artery. From these fa.cts it seems probable that in the younger snake embryos, of all species, both pulmonary arteries will be found present. If this is the case, the proof will be streng that, in all vertebrates with lungs the pulmonary arteries originate one from each pulmonary areh, and that Rathke's diagrams, thongh describsing perfectly the adu1t and 1ate embryonic conditions, are as regards this origin, incorrect.
- 1. Miiller’s Archiv, 1843, P. 276.
- 2. Anatomie des Frosohea diagram, P. 285.
- 3. ltiotsphologisches Jahrbuch 1901, p. 419.
In the Pig, one of the animals examined by Rathke, although the symmetrieaä origin is preserved, one pulmonary artery arising from each pujmonarsy Stroh, and although the nltimate appearanoe, that of both pulmonary arteries arising from the mid-point of the left piulmonary aroh, is the same as in the other mammals I have examined, the intermediate steps are different, as is shown in Figa 4 to 9. Instead of remaining comparatively parallel, as in the rabbit, the pulmonary arteries, after attaining considerable length (pig of 7.8 mm.), bend toward each other, and insteaci of remaining without branohes (exeept those deve1oped later in the lange) send out buds, each toward the other artery, Fig. 4, X, II. This bending toward the median line of these two pulmonaxry arteries is perhaps eaused by the great growth of the aurieles of the heascst in the Fig. Both processes continue until in a pig of 9 mm. there is at 1east one eonneetion between the right and Ieft pulmonary arteries, often two, as is snggested in Fig. 5, X, F, while in a pig of 11.0 mm. the two arteries, along a considerable part of their length, have merged into one channel, Fig. 6. Meanwhile the npper or proximal part of the right pulmonary arterzg which often shows signs of irregu1arity, such as a double origin, Figs. 4 and 7, D, ceases to increase in size, then grows sma11er, and soon becomes obliterated, so that all the blood to both lnngs Hows through the left puhnonary arterzn This gradual change is shown at D, Figa 5, 6 and 7, and in Fig. Z. where only the remains of the right artery are seen. For a 1ittle while after the obliteration of the lumen, a cord of connective tissue marks the former course of the right pulmonary artery, but soon even this disappears.
Fig. 5. Pig of 9.0 mm. Frontal series No. 54, Sections 462—502. x 80 diameters.
Fig. 6. Pig of 11.0 mm. Sagittal series No. 8, Sections 96-113. X 80 diameters.
Fig. 7. Pig of 12.0 mm. Transverse series No. s, Sections 366-404. x 80 d,ia-meters.
Fig. 8. Pig of 12.0 mm. Prontal series No. S, Sections 429-464. X 80 diameters.
Fig. 9. Pig of 20.0 mm. Frontal series No. 61, Sections 270-279. X 80 diameters
Along with this change, another, common to all mammals, has taken place, naInely, the obliteration of the right pulmonary archz but this is not the cause of the obliteration of the right pulmonary artery, since the lumen of the latter is the first to close, Fig. 8. Still another change is seen, as in the rabbit, in the lengthening of the truncus pulmonalis at the expense of the two pulmonary arches, and the oonsequent appars ent movement of the left pulmonary artery toward the right pulmonary arch. In the pig, considerable variation seems to occur in regard to the sta-ge of growth at which this last mentioned change takes place, as may be seen by comparing Figs 6 and 7, where the distance between the points of origin of the pulmonary arteries is about the same in two pigs of 11.0 and 12.0 mm., respectively, and Fig. 8, where the distance is much greater, although the length of the eInbryo is again 12.0 mm. 1t will be seen that of the two 12.0 Ihm. pigs, one still has, and one has already lost, the connection of the right pulmonary artery.
Whether all ungulates, or only pigs, have this odd method of arriving at the adult relations of the pulmonary arteries, I do not as yet know; certainly there is nothing like it in the rabbit, the cat, the dog, or in the human embryos within my reach.