470 
DE. PETTI GrEEW ON THE AEE AN GEMENT OF THE F1BEES 
usually consisting of two as in the left (Plate XIV. fig. 43, pp 1 ), are not necessarily limited 
to this number. In the camel and American elk the carne® column® are altogether 
wanting, the musculi papillares (two in number) being alone present. In the heart of 
the armadillo and red deer the carne® column® are feebly developed, the musculi papil- 
lares being generally three in number and small. In the heart of the sheep the carne® 
column® are more fully developed than in the preceding, the musculi papillares being 
sometimes two in number, and sometimes three (Plate XIII. fig. 19, h). In the heart 
of the pig, leopard, and calf (Plate XIY. fig. 43) the carne® column® are still more 
strongly developed, and appear in the form of thick muscular spiral ridges (o), which 
slightly intersect each other and cross the floor of the ventricle (r 1 ), the musculi papil- 
lares being sometimes two ( h h!), sometimes three, and sometimes four in number. In 
the porpoise, dugong, mysticetus, and human heart (Plate XIV. fig. 44) the carne® 
column® are more or less reticulated, particularly in the two latter, the musculi papil- 
lares varying from two to four ( h h! h" h!"). This increase in the number of papillary 
muscles in the right ventricle, which might at first sight seem to interfere with the 
bilateral distribution of the fibres in the primary or typical ventricle, is accounted for 
by the fact that in the right ventricle the muscular fasciculi, from which the papillary 
muscles spring, do not always coalesce as in the left ventricle, but remain permanently 
apart. The musculi papillares of the right ventricle are less distinctly spiral than those 
of the left, and are somewhat flattened to suit the concavo-convex shape of the right 
ventricular cavity. They occupy the septal wall posteriorly (h" h!"), and the right 
ventricular wall anteriorly ( h h!). In the right ventricle, as in the left, many of the 
fibres of the carne® column® (Plate XIY. fig. 43, o') are continuous at the base with 
corresponding external fibres ( d'f ), such of them as are not continuous, together with 
the musculi papillares, being rendered so by the intervention of the right auriculo- 
ventricular tendinous ring (Plate XIY. fig. 43, and Plate XY. fig. 46, n'), the chord® 
tendine®, and the segments of the tricuspid valve (Plate XIY. fig. 44, i'). 
Muscular valve of the right ventricle of the bird , how formed ? 
Distinction founded thereupon. In the right ventricle of the bird, where the tricuspid 
valve of the mammal * is supplied by a fleshy one, the continuity of the external with 
the internal fibres at the base is complete. This valve, as has been stated, forms the 
distinguishing characteristic between the ventricles of the bird and mammal, and differs 
essentially in its structure from all the other valves of the heart. 
It consists of a solitary fold of muscular substance (Plate XIY. figs. 38, 40, & 41, i), 
which extends from the edge and upper third of the septum posteriorly (Plate XIV. 
* The tricuspid valve, as its name implies, consists of three leaves or segments. As, however, the anterior 
segment, or that nearest the pulmonary artery, is larger than the posterior and internal segments, from which 
it is divided by a deeper notch than divides the posterior and internal segments from each other, some investi- 
gators regard the valve as consisting of two portions only ; and I am inclined to assent to this view, from the 
bilateral nature of the left ventricle, and from my conviction that the right ventricle and every thing pertaining 
to it is a segment of the left. 
