460 
DE. PETTIGEEW ON THE AEEANGEMENT OF THE FIBEES 
fig. 6) are the internal continuations of the twofold set of fibres forming the second 
external layer. They wind in two portions (o, n), in a spiral slightly vertical direction 
from right to left upwards, or from apex to base, so that they cross the fibres of the 
second layer at a somewhat acute angle. At the base they fold upon themselves in 
an outward direction, and form flattened bands ( n ' n"), which become continuous with 
corresponding bands belonging to the second layer (Plate XII. fig. 2 ,fg, d d! e). The 
amount of spiral made by the fibres of the sixth layer from the time they leave the 
apex until they reach the base, is rather under a turn and a half. Taken together, the 
fibres of the second and sixth layers embrace in their convolutions the fibres of the 
third, fourth, and fifth layers, they themselves being overlapped by the fibres of the 
first and seventh layers. 
Seventh or last internal layer of the left ventricle {Mammal). The fibres of the 
seventh or last internal layer (Plate XII. figs. 7 & 8), as was stated when describing 
the first or superficial layer, form the carne® column® and musculi papillares, especially 
the latter. When the two sets of fibres which constitute the superficial or first external 
layer are traced from above downwards, the anterior set, as was explained, is found to 
enter the apex posteriorly (Plate XII. fig. 10, d), to become continuous with the fibres 
of the anterior musculus papillaris (Plate XII. figs. 12 & 13, y) and those of the came® 
column® next to it — the posterior set entering the apex anteriorly (Plate XII. fig. 10, g), 
and becoming continuous with the fibres of the posterior musculus papillaris (Plate XII. 
figs. 12 & 13, x) and the adjoining carne® column®. The musculi papillares and 
carne® column® therefore occupy a variable position in the interior of the ventricle, — 
the anterior musculus papillaris winding in a spiral almost vertical direction from below 
upwards, and from right to left, from the interior of the apex posteriorly to occupy an 
anterior position ; the posterior musculus papillaris winding in a corresponding direction 
from the interior of the apex anteriorly, and curving round to occupy a posterior position. 
The musculi papillares, as will be seen from this description, are not the simply vertical 
columns usually represented (Plate XII. fig. 14, x,y ), but vertical spiral columns (Plate 
XII. fig. 13, x , y). The musculi papillares are seen to advantage in the ventricle of 
the giraffe, camel, lion, horse, ox, ass, deer, seal, and dog. They occur in various stages 
of development*. 
Most commonly they appear as conical-shaped spiral bodies, which project into the 
ventricular cavity and extend, in moderate-sized hearts, from the extreme apex to within 
half an inch or so of the base. At the apex, where they may be said to originate, and 
where they project least into the cavity, their spiral nature is very distinct f. 
Towards the base, where their spiral direction is less marked, and where they project 
most into the ventricular cavity, they terminate in free blunted extremities, which are 
obliquely cut from below upwards, and from within outwards. As the chord® tendine® 
connect the blunted extremities in question with the segments of the bicuspid valve, 
* In the American elk (Plate XY. fig. 48, y) they are so rudimentary as scarcely to attract attention. 
t To see the spiral course pursued by the musculi papillares, the ventricle should he opened anteriorly, the 
incision being carried not quite to the apex, as represented at Plate XII. fig. 13, x y. 
