IN THE VENTRICLES OF THE VERTEBRATE HEART. 
489 
tion, or by the aid of diagrams 4 & 5, the fibres of figs. 8 & 7 crossing those of fig. 1, 
the fibres of fig. 6 crossing those of fig. 2, and those of fig. 5 crossing those of fig. 3. 
The minutely reticulated structure to which this disposition of the fibres gives rise, 
although very simple when the layers are regarded separately or apart from each other, 
becomes very perplexing when they are placed in apposition or as they occur in the 
undissected ventricle ; and to the partial dissection of the layers perhaps more than to 
any other cause, is to be attributed that numerous class of complicated diagrams 
which represent the fibres of the ventricles as running in all directions without either 
law or order. In those diagrams that beautiful gradation in direction by which the 
fibres diverge from an imaginary vertical, and gradually return to it after having inter- 
sected each other in all directions, finds no place. In conclusion, the scheme of the 
course and direction of the fibres as summed up, while it greatly facilitates the com- 
prehension of the general principle involved in the ultimate structure of the ventricles, 
harmonizes in the most perfect manner with all that is at present known of the heart’s 
movements — those movements apparently so simple, and yet so difficult of analysis. 
Explanation op the Plates. 
In the engravings the same letters have been employed, as far as possible, to designate 
corresponding portions of the ventricles. The description of the figures in Plate XIII. 
from 22 to 29 inclusive does not follow in strictly numerical order. Thus the description 
of fig. 24 follows the description of fig. 22, and precedes that of figs. 25, 27, & 28 ; while 
the description of fig. 23 follows that of fig. 28, and precedes that of fig. 26. The object 
of this arrangement is to ensure that the description may be read as a connected narra- 
tive. In the figures of the right ventricle some of the layers have not been represented, 
from a desire to curtail the number of figures. This, however, can occasion no diffi- 
culty, as portions of the unrepresented layers may be seen in other figures. A portion 
of the second external layer of the right ventricle (one of the omitted layers) is seen at 
f of fig. 22, while the greater portions of the two other unrepresented layers, viz. layers 
four and seven, are seen at Plate XIII. fig. 23, p, q, and Plate XIV. fig. 43, o, h. 
PLATE XII. 
Fig. 1. Left ventricle of the sheep’s heart, seen posteriorly. Shows the superficial or 
first external layer. See pages 454, 455, & 456. 
Fig. 2. Left ventricle of the sheep’s heart, seen posteriorly. Shows the second layer. 
See pages 456 & 457. 
Fig. 3. Left ventricle of the sheep’s heart, seen posteriorly. Shows the third layer. 
See page 458. 
