IN THE VENTRICLES OE THE VERTEBRATE HEART. 
481 
internal fibres changing their direction at this point to become external. On the right 
ventricular wall the fibres composing the more central layers are distinctly seen to cross 
each other (f). 
2ndly. The fibres which are common to both ventricles posteriorly, can be traced pass- 
ing from the left ventricle to the right (g). 
3rdly. The large fascicular bundles of fibres which constitute the camese column® 
(oo'o"), and the free ends of the musculi papillares with their chord® tendine® attached 
(xy), may be recognized surrounding the ventricular cavities ( b l). 
4thly. The shape of the left ventricular cavity at this point is oval ( "b ), and of the 
right cavity concavo-convex ( l ). 
5thly. The thickness of the left ventricular wall (c d) and the septum (c'd 1 ) is as nearly 
as possible equal, that of the right ventricular wall ( c"d ") being only half the thickness 
of either of the former. 
6thly. The thickness of the ventricular walls, as a whole, is not quite so great in this 
section as it is three-quarters of an inch nearer the apex (Plate XV. fig. 50). 
In a transverse section rather less than an inch and a half from the base (Plate XV. 
fig. 50) the same phenomena are repeated, with the following slight differences: — 
1st. The aggregate of the external fibres ( c c") is considerably less than the aggregate 
of the internal ones ( d d"), out of which the carne® column® (o) and musculi papillares 
(x y, hh') spring — an arrangement necessitated by the external fibres requiring in this 
instance to crowd together, in order to accommodate themselves to the diminished 
calibre of the cone interiorly. 
2ndly. The common fibres posteriorly (g) pass from the left to the right ventricle, and 
dip or bend in at the track for the anterior coronary artery, to become continuous with 
fibres having a similar direction on the septum (m). 
3rdly. On account of the great preponderance of the internal fibres, and the project- 
ing of the musculi papillares into the interior, the appearance of the ventricular cavities 
is considerably changed. Thus the left ventricular cavity (b) is triangular — the right 
ventricular one ( l ) being concavo-convex, and having two constrictions init caused by the 
anterior (b!) and posterior (A) musculi papillares. 
4thly. The thickness of the septum (c'd') is nearly one-sixth greater than that of the 
left ventricular wall ( c d), between the papillary muscles (xy), the thickness of the right 
ventricular wall ( c"d ") being only half that of the latter. 
5thly. The thickness of the ventricular walls is greater in this section than in any 
other. 
When a transverse section is made about two inches and a quarter from the base 
(Plate XV. fig. 51), the thickness of the ventricular walls ( cd , c'd', d'd") is found to have 
diminished slightly. The peculiarities of this section are these : — 
1st. The preponderance of the internal (d d") over the external (c c") fibres, especially 
in the left ventricle, is more marked than in the two preceding sections. 
2ndly. The circular nature of the more central fibres is also better defined (e e'), the 
fibres being observed to cross each other (/). 
