DIAPHRAGM 



243 



laterally into the pleuro -peritoneal membranes, causing an uplifting of their 

 ventral pillars (cf. fig. 295) and drawing-in of the outer and ventral margins of the 

 openings. The ultimate result is the merging over the surface of the expanding 

 liver on each side of the mesial layer of the pleuro-peritoneal membrane with 

 the layer covering the thickened mesenteric septum (the mesolateral fold being 

 considered part of this septum). 



The primitive diaphragm is thus formed first of the septum transversum 

 (including under that term a part of the pleuro-pericardial membrane) ; second 

 of the thickened mesenteric septum, and third of the mesial layer of the pleuro- 

 peritoneal membrane. It consists at first merely of a connective-tissue sheet 

 covering the liver ; it is at first continuous with the parenchyma of the liver, but 

 soon becomes separated from it by cleaving into^two layers. The cleavage, how- 

 ever, does not extend all round the organ, and jthe/esult is that it remains attached 

 by suspensory bands, which afterwards become the coronary ligaments. 



The diaphragm thus consti- 

 tuted is placed at first very 

 obliquely, but as it descends to 

 its definitive position it comes 

 to lie transversely, and at the 

 same time increases in circum- 

 ference by the expansion of the 

 pleural spaces. These, as we 

 have already seen, are at first 

 small, and are placed entirely 

 dorsal to the pericardium. Owing 

 to the development of the ribs 

 the body -wall now becomes 

 greatly expanded, and a thick 

 layer of loose tissue develops on 

 their inner side (fig. 268, p. 214). 



This is gradually invaded by the FlG< 298 ._ DlAOBAM OF THE PRIMITIVE DIAPHRAGM TO 

 pleural sacs, which, pushing into SHOW THE SEVERAL PARTS FROM WHICH IT is BUILT UP. 



the body-wall round the peri- (After Broman.) 



cardium, gradually come to en- 1, pericardial part derived from septum transyersum; 



' J 2 and 3, parts derived from the mesentery : 4, 4, parts 



Close that cavity, and extending derived from pleuro-peritoneal membranes : between these 



dorsally and the mesenteric portions are the nearly closed 

 leuro-peritoneal openings ; 5, 5, parts derived from the 



fly-walls ; 1, 2, 8, 4, 4, cover the cranial aspect (upper 

 rfa< 



behind into the tissue circum- 

 scribing the diaphragm form 

 the costo-diaphragmatic recesses, surface) of the liver. 

 The pleural sacs thus reach their 



final dimensions by excavating the body-wall, and, further, a certain portion of 

 the circumference of the diaphragm must be referred to tissue really belonging 

 to the body- wall (fig. 298). 



Muscular tissue extends into the diaphragm derived from the transversalis 

 and the rectus sheets (Keith). The supply of the muscle by the phrenic nerves 

 shows that part of it is a derivative of cervical myo tomes, which are displaced 

 backwards as the diaphragm sinks to its permanent level. 1 



1 The above account of the development of the diaphragm is a mere sketch of a very complicated 

 process. For further details the reader must be referred to special works on the subject. The earlier 

 literature will be found fully reviewe<i by Brachet in Merkel and Bonnet, Ergebnisse der Anatomie und 

 Entwickelungsgeschichte, 1907. References to some more recent papers will be found in Hochstetter's 

 article in Hertwig's Handbuch, Bd. Ill Th. i. and ii. p. 160. Still more recent papers are Mall, Ball. 

 of the Johns Hopkins Hospital xii. ; Broman, Anat. Anzeiger Ergiinzungshef t) xxi. ; Bertelli, Arch. Anat. 

 e Embriol. Ital iv. ; Keith, Jour, of Anat. and Phys. xxxix. ; Wiilfel, Anat. Anzeiger xxx. ; Debeyse, 

 Bibliograph. Anat. xiv. ; Brachet, Contribution a la signification morphologique du diaphragm dorsal, 

 Bruxelles 1906. 



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