IN THE VENTEICLES OF THE YEETEBEATE HEAET. 
463 
ventricular cavity {b) is again at right angles to the long axis of the cavity of the 
right ventricle, making it evident that the cavity of the left ventricle has made a full 
spiral turn; and in the fifth section (Plate XV. fig. 53) the long axis of the left ven- 
tricular cavity is again parallel with the long axis of the cavity of the right ventricle, 
showing that the left ventricular cavity has made an additional half turn. If another 
section had been made at the extreme apex, probably an eighth of a turn more would 
have been obtained, as the spiral in this direction is very rapid. The object of the cavity 
twisting suddenly upon itself at the apex is obviously to protect the ventricular wall, 
where thinnest, from undue pressure; for it is plain that a fluid injected into a conical- 
shaped spiral and therefore tortuous cavity will not be transmitted to the apex with the 
same degree of force as it would if the cavity were not spiral. 
Vertical section of the left ventricular wall {Mammal). 
On making a vertical section of the left ventricle between the musculi papillares 
(Plate XII. figs. 13 & 14), the ventricular wall (s), like the ventricular cavity, is observed 
to form a double cone, the apices of which point towards the apex and base of the ven- 
tricle respectively, the bases, which are united in the upper portion of the middle third, 
corresponding with the thickest part of the ventricular wall. The varying degree of 
thickness in the ventricular wall is traceable to the fact, that the outermost and inner- 
most layers extend further towards the apex and base than those which come next, and 
these, again, further than those which succeed them, and so on until the central layer is 
reached — this being of least extent, and confined indeed to about the middle third of 
the ventricle. Thus into the apicial portion of the ventricular wall, where thinnest, only 
one layer enters, viz. the superficial or first external*. Into a second portion, a little 
above the apex, two layers enter, the first and the seventh ; into a third or higher portion 
four, viz. the first and the seventh, the second and the sixth ; while into a fourth, or still 
higher portion, which corresponds with the upper part of the middle third, the whole 
seven layers enter. Tracing the thickness of the ventricular wall in an opposite direc- 
tion, i. e. from the base towards the upper part of the middle third, the same changes 
present themselves, although in a less marked degree. Thus the extreme base consists 
of two layers, the continuations in fact of those forming the second part of the apex ; 
the second portion of four, the third of six, and so on — an arrangement which accounts 
for the ventricular wall being thicker towards the base than towards the apex f. 
Recapitulation of facts connected with the left ventricle {Mammal). Before leaving 
the left ventricle, it may be well to recapitulate briefly the points more particularly 
dwelt upon. They are the following : — 
* Tins portion of the ventricular wall is formed by the external fibres turning round in a circular direction, 
to alter their direction and become continuous with the internal, the external and internal fibres, in virtue of 
their spiral direction, not crossing each other until removed an appreciable distance from the apex. 
t The septum also tapers in two directions, more particularly when, as in the present demonstration, it is 
regarded as forming part of the left ventricle. 
3 Q 2 
