454 
DE. PETTIGEEW ON THE AEE AN G-EMENT OE THE EIBEES 
2nd. The varying direction of the fibres on the body of the ventricle, produced by the 
different angles which the fibres of the several layers make with the imaginary vertical 
referred to. 
3rd. The double entrance of the fibres at the apex, and their exit in two sets from 
thq left auriculo-ventricular opening. 
4th. The manner in which the apex is opened into, and enlarged, by the removal of 
successive layers. 
5th. The gradual increase in the thickness of the layers when the dissection is con- 
ducted from without inwards. This is in all probability owing to the more internal or 
deeper layers being the first formed. Thus the seventh or deepest internal layer, which I 
am inclined to think is developed before the sixth, goes on increasing pari passu with 
it ; while the seventh and sixth increase equally with the fifth, which is a later forma- 
tion ; and so on until the first, which is the thinnest layer, is reached. That this 
explanation has its foundation in truth, is probable from the fact that, in imitating the 
process by which I believe the left ventricle is formed, the seventh or most internal 
layer supplies a basis of support for the more superficial layers — -just in the same way 
that the smaller and more central turns of a shell form the basis of support for the 
peripheral or more superficial turns*. 
External layers of the left ventricle of the Mammal. 
Superficial or first external layer. On looking at the left auriculo-ventricular opening 
of the sheep and calf posteriorly (Plate XII. fig. 1, b, and Plate XV. fig. 46, b), when 
the serous membrane, fat, vessels, and nerves have been removed, the fibres are seen to 
issue from it in fascicular bundles, and to curve over its margin all round (d,f). 
The fibres on leaving the opening lose to a considerable extent their fascicular 
character, and naturally arrange themselves in two sets, — the one set proceeding from 
the anterior portion of the opening ( d ), and the anterior half of the septum (e) ; the other 
from the posterior portion of the opening ( f ), and the posterior half of the septum (g). 
On the body of the ventricle the fibres spread out to form a smooth muscular sheet, 
both sets pursuing a spiral nearly vertical direction from left to right of the spectator. 
In their course they gradually change their position on the ventricular wall, the fibres 
from the anterior portion of the opening and the anterior half of the septum winding 
round and appearing on the posterior surface , the fibres from the posterior portion of 
the opening and the posterior half of the septum, on the contrary, winding round and 
appearing on the anterior surface. On nearing the apex, which they do in graceful 
curves (Plate XII. fig. 9), the two sets of fibres become more strongly defined, the fibres 
of either set converging and overlapping to a greater or less extent. At the apex 
both sets ( fg , e d) curve rapidly round and form a whorl or vortex of great beauty, — the 
fibres from the anterior portion of the opening and septum curving into those from 
* For a detailed account of this view, see p. 484. 
