IN THE VENTRICLES OF THE VERTEBRATE HEART. 
473 
layer of the right ventricle of the mammal as are not continuous with corresponding 
external fibres at the base arise in two divisions (Plate XIV. fig. 30, d,f), — the one from 
the fibrous ring surrounding the pulmonary artery (Plate XV. fig. 46, Jc) and aorta (a), 
and the anterior half of the fibrous ring surrounding the right auriculo-ventricular 
opening (l), together with a corresponding portion of the septum (<?) ; the other from the 
posterior half of the fibrous ring ( n ') surrounding the right auriculo-ventricular opening, 
the posterior half of the septum, and a limited portion of the left auriculo-ventricular 
tendinous ring posteriorly ( n ). In this layer, consequently, comparatively few of the 
fibres belonging to the left ventricle (Plate XIII. fig. 18,/) cross the posterior coronary 
groove (j) to become continuous with the fibres on the right (/') ; and it is worthy of 
observation, that as the dissection advances the number of the so-called common fibres is 
augmented. This increase of the common fibres, which is gradual and follows a certain 
order, is referable to the source and direction of the fibres constituting the several layers. 
In the first layer, as has been explained, the common fibres proceed from a limited por- 
tion of the left auriculo-ventricular opening posteriorly ; and as their direction is little 
removed from the vertical, few of them cross the posterior coronary groove to appear on 
the right. In the second layer, however, the common fibres proceed from the posterior 
and outer portion of the left auriculo-ventricular opening (Plate XIII. fig. 21, /), and, 
their direction being more oblique, a considerable proportion cross the posterior coronary 
groove (j ). In the third layer (Plate XIII. fig. 24, f d!) the direction is still more 
oblique, and a greater number of the fibres consequently cross the groove referred to. 
In the fourth layer the direction of the fibres is horizontal (Plate XIII. fig. 27,/ d), and 
the fibres almost all cross the groove in question. In the last-mentioned layer the fibres 
may be said to emanate from the left auriculo-ventricular opening all round. In speaking, 
therefore, of the fibres which are common to both ventricles posteriorly, it will faci- 
litate the comprehension of their arrangement, to say that they radiate from different 
portions of the left auriculo-ventricular opening at different levels, these levels corre- 
sponding with the depth of the layer involved. 
Peculiarities of the right ventricle of the mammal— fleshy pons — infundibulum — bone 
of the heart , &c. In the right ventricle, as in the left, the layers increase in thickness 
from without inwards ; but there is this difference : the layers of the right ventricle are 
comparatively thinner than those of the left, owing to the fibres constituting them being 
more delicate. The greater delicacy of the fibres of the right ventricle may be explained 
either by an arrest of growth after birth, or to their becoming subsequently atrophied. 
The fibres of the right ventricle, as a rule, form only curves or segments of spirals 
(Plate XIV. figs. 36 & 37), a certain number of them anteriorly (especially those of the 
internal layers *) bending over and uniting with corresponding fibres from the right side 
of the septum to form a fibrous archway (Plate XIII. figs. 18, 21, 24, & 27, to), which 
* The fibres of the external layers which enter into the formation of the fleshy pons, arise in many instances 
from the root of the pulmonary artery and aorta, and from the anterior portion of the right auriculo-ventricular 
opening. 
