500 
DE. PETTIGEEW ON THE EIBEES OE THE VEETEBEATE HEAET. 
ventricle, in imitation of the constructive process in the embryo, a double 
septum unattached posteriorly (B) is produced, this septum dividing the 
ventricle into two, a right or rudimentary ventricle (D), and a left or more 
complete one (C). It also shows how the fibres may be continuous or common 
to both ventricles posteriorly (B), while anteriorly (A) they dip in at the 
track for the anterior coronary artery and are to a certain extent independent 
of each other (Plate XV. fig. 50, compare with g and m). See pages 464, - 
465, 466, 467, & 488. 
Diagram 16 shows how the posterior fold (G) of the septal duplicature, by passing 
through the posterior wall (B) until the central layer in either is reached, 
completely isolates the right ventricle (D) from the left (C), and how, by 
the atrophy of the right ventricular wall (F) and its share of the septum (K), 
after birth, to half their original dimensions, the right ventricle (F) is reduced 
to half the thickness of the left (I) ; while the septum (H K) is three times 
as thick as the right, and a third thicker than the left. See pages 465, 466, 
467, & 488. 
Diagram 17 shows how, by the partial absorption of the posterior fold of the septal 
duplicature (K), and the passing through and blending of the right half of 
the reduplication (E) with the left (H), the ventricles are more intimately 
united, and the septum reduced until it is only a sixth greater than the left 
ventricle, — an arrangement which very nearly corresponds to the measurement 
of the actual septum between the musculi papillares at the thickest part 
(Plate XV. fig. 50, compare d d 1 with c d). It also shows how the septum 
is composed of two elements, and how one portion of it (E E) belongs exclu- 
sively to the right ventricle, a second portion (H) to the left ; while a third 
portion ( J J) belongs partly to the one ventricle, and partly to the other. See 
pages 465, 466, & 467. 
Diagram 18 shows how the right portion of the septum, as seen at EE of diagram 17, 
may become absorbed, so as to reduce the septum (H) to the thickness of 
the left ventricular wall (I) and half the thickness of the right (F) (Plate XV. 
figs. 49 & 51, compare c d with d d\ and c d , d d', with d 1 d"). 
