IN THE VENTEICLES OE THE VERTEBRATE HEART. 
455 
the posterior portion of the opening and septum, and entering the apex posteriorly 
(Plate XII. fig. 10, d), to become continuous with the fibres of the carne® column® 
and anterior musculus papillaris (Plate XII. fig. 13, y ) ; the fibres from the posterior 
portion of the opening and septum curving into those from the anterior portion of the 
opening and septum, and entering the apex anteriorly (Plate XII. fig. 10, g), to become 
continuous with the fibres of the carne® column® and posterior musculus papillaris* 
(Plate XII. fig. 13,#). 
The fibres, therefore, which issue from the auriculo-ventricular orifice anteriorly,- enter 
the apex posteriorly, and vice versa — an arrangement which is accounted for by the fibres 
of the superficial or first external layer, from the time they leave the base until they 
reach the apex, making one turn and a half of a spiral. As the fibres of the carne® 
column® and musculi papillares, which constitute the seventh or last internal layer, 
also make a turn and a half, from the time they leave the apex until they reach the base, 
the external and internal fibres always return to points not wide of those from which 
they started. It was, no doubt, this circumstance which induced Lower and Gerdt 
to describe the external and internal fibres as forming a more or less perfect figure 
of 8f, these investigators differing as to the completeness of the figure from having, in 
all probability, described different and deeper layers J. The bilateral distribution of 
the fibres, which extends to all the layers, has hitherto escaped observation, but is 
clearly established by my dissections. Viewed in connexion with the musculi papillares 
and the segments of the bicuspid valve, it is, as I shall endeavour to show, of consider- 
able physiological importance. The object of the two sets of fibres curving into each 
other at the apex §, is evidently threefold : first, to secure symmetry, structural and 
* The fibres of the carnese column se and musculi papillares pursue a spiral nearly vertical direction, from 
right to left upwards, so that they cross the fibres of the superficial or first external layer ; for an explanation 
of the course and direction of the fibres of the first and seventh layers, see A, B, C, D, E of diagrams 3 & 4, 
Plate XVI. 
t Wmsiow altogether, though wrongly, ^denied the crossing of the external and internal fibres (Memoires 
de l’Acad. Roy. 1710, p. 197). 
t Great assistance may be obtained in comprehending the scheme of the arrangement of the fibres, by occa- 
sionally referring to the diagrams contained in Plate XVI. In these diagrams the fibres are represented by lines 
drawn at intervals, the object being to furnish the reader with a transparent ventricle, which will enable bim 
to analyze its structure by tracing the fibres composing the several layers throughout their entire extent. Thus 
at A A' of diagram 4, the fibres of the superficial or first external layer are indicated, the fibres of the seventh 
or last internal layer being seen at E E' of diagrams 4 & 6. In diagram 5 the fibres of the second layer are 
represented by the lines marked B B', the fibres of the sixth or corresponding layer being marked D D'. In 
diagram 4 the fibres of the third and fifth layers are marked B and D, and the fibres of the fourth or central 
layer are seen at C of diagrams 4 & 6, and at C C' C” of diagram 5. In diagram 3 the lines are drawn at still 
wider intervals, and show how the external fibres A, B, become internal (D, E) by turning upon themselves at 
"the apex C, where they also enter the interior. Diagrams 7 & 10 show how the external fibres A B, C D enter 
the apex in two sets at opposite points, viz. at E and E, while diagram 11 shows how the internal fibres are 
arranged in two sets (X and V) in the interior. 
§ The auriculo-ventricular orifice at the base is also closed by two symmetrical structures, viz. the anterior 
and posterior segments of the bicuspid valve. 
3 p 2 
