On the so-called interpleural opening in the opossum didelphys virginiana (Kerr).код для вставкиСкачать
ON THE SO-CALLED INTF’IRPLEIJRATJ OPENING Ir\; THE OPOSSUM, DIDELPHYS VIRGTNTRXA (KERR) TNTROT)UCTIOR’ The obseisvations 1-epartedin tliis paper were niacle clwing the study of t i n miclescyibeil stage of pouc~li young of thv opossum, part of a prograin to complete infoi*matioii coimi‘iiing Didelpliys virginiana ( K e r r ) , nndcrtakcn by the Zoology D e p y t m e n t of the University of Pennsylvania and The TVistar Institute of Anatomy and Biology. The author is indebted to The Wistar Institute for permission to study their slides, and to Dr. Olin E. Nelsen for the loan of serial sections, and for specimens of all ages for dissection. Various authors, basing their opinioiis on reported morpliological peculiarities, have considered tliat the opossum differs fro111 typical maiiimals, arid liave coiist ructed hypotheses t i s io the origin of the Eutlicria, and us to the position of the Narsupialia with respect to the Saul-opsida and the Eutheria. One of the regions mentioned most frequently as being atypical, is the thoracic region. I t has been reported (Jazuta, ’32 ; Hartman-Weinbcrg, ’24) that the pleural cavities in Didelphys communicate with each other by means of the socalled ‘interpleural opening. ” I n this paper it will be demoiistrated that no such opening between the right and left pleural cnvities exists in Didelpliys virginiana, and that the plenrtil cavities of the opossum resemble markedly those of other ri~arnnialsnot closely related to Ihc mai*supials. ( r,87 OBSERVATIONS OR’ DIDELPHYS VIRGTNIANA As has been noted by various authors, the left lung of the opossum has only one lobe. The right lung has in addition to the usual three lobes, a fourth lobe, called the accessory or infracardiac lobe. I t has been noted, too, that the right lung overlaps the heart, and that the ventral part of the mediastinal septum runs diagoiially towards the left of the median line of the body; in fact, it joins tlic diaphragm to the left of tlic pcricurditil cavity (fig. 2 A). Several investigators have observed that the oesophagus is long in Didelphys, especially in its ventral portion. I t s course is diagonal, from dorsornedial in the cephalic portion of the thorax t o more ventromcdial where it passes through the diaphragm. As a result, there is dorsad to the oesophagus a triangular space bounded rouglily by the oesophagus, aorta, a i d diaphragm. Across this is stretched the thin mediastinal septum, composed of R double layer of splanchnic pleura. It is in this region that the interpleural opening is supposed to be. I n dissections and scrial sections of embryos arid of 10- arid 15-day-old pouch young the author could find no interpleural opening. I n all specimens the right and left pleural cavities were completely separated (figs. 1, 2). Older stages were then examined: pouch young of 50 days, young of 100, 109, 150, and 185 days of age, and g number of young adults of unknown age. I n no case was there found to be any cornmunications between the right and left pleural cavities. I n all the specimens studied, the thin mcdiastinal scpt um was intact from the oesophagus to the aorta, and from the aortic arch to the diaphragm. One peculiarity of the thoracic region seems to have escaped notice; that is, the infixcardiac lobe of the right lung often lies in a separate cavity, a pocket iii the mediastinum. Thus, especially in young opossums, tliere may he three pleural spaces (figs. 1 and 2). The pocket in which the accessory lobe lies is caudad and ventrad to the heart. On the left siclo it is separated from the left pleural cavity lip the ohliqnc septum nictliastinalis. A fold SO-CALLED 1NTEHPLEUl:AL OPENkNG 589 of pleura scparates the main part of the right luiig from the accessory lobe either coniplctely or partially. As this pleural fold courses ventrad it covers the postcava and coritiiiueb ventro-caudally to the diaphragm. This partition may be incomplete dorsally, between the postcava and the oesophagus, or ventrally, where it may fail to join the body wall o r thc pleural sac around the main part of the right lung. Caudoventrally the cavity lies between the cliaphra-gm and the pericardial sac. Thus, in a ventral clissectioii of the thoracic Fig. 1 Diagrammatic drawing of tranvrersc scctioii through heart region of 15-day pouch young. A, mediastinum. B, plcural cavity of left lung. C, pocket where the infraeardiae lobc lies. D, heart. Figure viewed looking caudad. Fig. 2 Diagrammatic drawing of transversc section through region caudad t o hcart of 15-day pouch young. A, ventral p a r t of mediastinal septum. B, infracardiac lobe of tlic right lung. C, fold of pleure forming p a r t of pocket i n which the infracardiac lobe lies. E, postcava. I), p a r t of diaphragm. Figure viewed looking caudad. region, a coiiipartment niay sometiiiies be seen below the heart, in which is visible the tip of the accessory lobe of the right lung. The presence of an accessory lobc to the right lung lying completely or partially in a pocket in tlie mcdiastinum is not peculiar t o the opossum. 111 the rat the right luiig possesses ail accessory lobe which is notched to perniit passage of the postcava. This lobe is called the postcaval lobe. As in the opossum, the left lung is not lobed and the mediastinal septum, ventrally, is diverted to the left of the pericardial sac. The greater part of the postcavd lobc lies caudad to the heart in a membranous pocket of the niediastinum, closed completcly tlic loft pleural cavity but open into tlic right pleural cavity dorsad to the postcava. In tlic cat t1iet.c is an accessor? niedian lobc of the right lung, called the mediastinal lobe. It is divided into two alriiost equal parts, the postcava passing bctween then). Thc left half of the mediastinal lobe lies in a pocket in the niediastinum dorsnd aiid caudad to the heart. This pocket is closed completely from the left pleural cavity. As in the opossum, the membrane separating tlic pocket from the right pleural cavity is open between the postcava arid thc ocsophagus. fro111 DISCUS810N hInch of the theoretical discussion as to the origiii aiicl evolution of the mamnials is based on reported anatoinical peculiaritics in the marsupials. In rccent literature (IlcCrady, '38) one finds the following: Jazuta ('32) observed tliat tlie right a d left pleural cavities of the opossum coniiriunicate with cach otlicr in the dorsomedial linc. This interpleural opening has bceii found in I)ii(clpliys, Pcrameles, and various birds. !"is fact . Jaxuta regards as sti*oiig cvitlencc that the rnai*supials represent a connecting link bctwccn the Euthcria and tlic Sauropsida. . . In tlie article cited by hlcCraciy, Jazuta writes: Hinter Clem Herzbeutel weisen dic Rrustfclle gaiiz bezonderc Verhaltnisse auf, und zwar : beginnend von 4 Brustwirbel bis zuni 13 liaben beide mediastinal-pleurae eine gcmeinsamc,liingsgcstrecktc 18 mrn. lang 6ffnung, die durch folgende Gcbilclc begrenzt ist : Vorne durch das Rrustfell, das die Spciscrolirc bekleidet, hiiitcii durch das dic Aorte bedeckelide Bt*ustfell, oben durdi den Aortwbogen, d e r sicli von €Icrxeii riickwiirts zur Miirbelsaiilc biegt, uiitcii dui.cli ciiic sicltelformige serosc Falte, die von der tlbsteigcriden Aorte zuiii Zmcrclifell geht. Die Entstehung dieser Offnung kanii man sic11in solchen MTeise vorstelten, dasz die beiden Ei-ustfclle zuerst zwischen der Aorta und der Speiserolire verwachsen maren, dann entstand aber 15ings ihrer groszten Strecke eine Koniiiiunikation zwischen beiden Pleurahohlen. SO-CALLED 1NTEItPLEURAL OI'ENIh-G 591 Jazuta refers t o an earlier paper by Hartnian-T~reiiiberg, describing an opening between the pleural cavities in several species of iiiarsupials. He gives tlie riamc ' ' intcrpleural opeiiing" to this, stating Bei Didelpliys marsupialis uiid Peramclcs Garragussi komiiiunizieren die Pleurahohlen vermittelst einer besondern Oeffnung in der Pleura inediastinalis,. die als Interpleural Offnung bczcichnet werden lrann. na bcigenannten Tieren dic rechte und linke mediastirialpleura unter Bildung einer Septum mediastinale mitcinandcr rcrwacliscii, flicssen dict hiedcn Plcuraloffnungen elmifalls zu ciilem eiiizigeii niil ciiiaridcr zusammen. On the basis of these observalioiis it lias been acceptcd that the pleural cavities of the opossum ciiK'w froiii the typical closed pleural cavity of Jlammalia. None of the speciiiieiis exaiiiined during the studies rcportccl in this paper showed a lengthwise split in the septum me&astirialis connecting tlie two pleural cavities. It is evident from these studies that such ail orxiling clocs not exist. The pleural cavities of Didelpliys \-ii*giniaiiaN re iiot atypical, but conforin t o tlic mammaliaii type of closctl 1)lculal cavities. Iforeover, there is a striking similarity amongst tlii*cv i q m sentatives of widely separated groups of mamiiials in tlic possession of an accessory lobe to the right lung. The opossum, the rat, and the cat have such an accessory lobe lying in tlic median thoracic region dorsad and caudad to tlic pericardinl sac. TI] each of these animals this lobc is honscd complc!tc~lyo r 1)artially in a special pocket in the niediastiiiuiii, septrutecl froiii the left pleural cavity by the ventral part of the scptuiii mediastinalis, communicating with the right pleural cavity by an opening in the pleural fold dorsad to the postcava. Careful comparative studies of other reported anatomical peculiarities of the opossum need to be niade. 592 ALICE M. IlUSSELL SUMNARP 1. No so-called “interplcural” openings were found in pouch young nor in older stages of Didelpliys virginiana. 2. The infracardiac lobe of the riglit lung lies in a special cavity in the mediastinum. 3. There is a striking siniilarity between the opossum, the rat, and the cat in the possession of an accessory lobe to the right lung, and in the presence of a special cavity in thc mediastinurn in which it lies. LITERATTJI~E CITED I I A R T ~ I A N - W ~ I N A. ~ ~ : ~1924 ~ ~ ; , ~ I l t c ~ ~ l J ~ ~ ~ i ~ ~ und l ~ f f nLuftsackc Ullg bpi Marsupialiern. Cotnpt. Belid. de l ’Acadcinic des Srirnees de Hussie 1924, pp. 26-29. JAZUTA, KONSTANTIN1932 13aubesonderlieiten der Plcur:ihOl~lei~bciui Opossuni. Anat. A m . , vol. 73, pp. 375-380. MCCLURE,C. F. W. 1902 The anatomy and developincnt of the posterior wnx cava in Didelphis virginiana. B i d . Bull., vol. 3, pp. 333-335. MCCRADY,EDWARD, JR. 1938 Embryology of the Opossuiu. American hnatonlical Memoirs, No. 16.