66 
DR. PETTIGREW ON THE ARRANGEMENT OF THE FIBRES 
ventricle ( f f "); whereas in front, with the exception of a large cross band at the base 
(Plate XIII. fig. 20, n), which is evidently for the purpose of binding the ventricles 
more securely together anteriorly, the fibres of the right and of the left ventricle 
respectively dip in (Plate XIII. fig. 23, r ) at the anterior coronary track (o o'), as if 
altogether independent of each other. 
3rdly. When the fibres on the anterior aspect which belong to one or other of the 
ventricles are traced into the septum, and the ventricles forcibly separated (Plate XIV. 
fig. 45) in a line corresponding with the course which the fibres peculiar to each ven- 
tricle naturally take, the right ventricle (Jc) claims, as its share of the partition alluded 
to, rather less than one-third of its entire breadth ( l ), the remaining two-thirds (n) 
going to the left ventricle (m). Why the right ventricle should claim less than a third 
of the septum is difficult to explain, unless it be that this portion of the septum, belong- 
ing as it does more particularly to the right ventricle, represents the right half of the 
septal fold atrophied to half its original dimensions (Plate XVI. diag. 16, K), in common 
with the other portions of the right ventricular wall (F) *. The right ventricular wall 
after birth, it will be remembered, is only half the thickness of the left (Plate XV. 
compare c"d" with c d of fig. 51). This view seems probable from the fact, that the sep- 
tum in some places (Plate XV. fig. 50, c'd ') is nearly a third thicker than the left ven- 
tricular wall (c d) between the papillary muscles (x, y ) — an excess in breadth which very 
nearly corresponds with what would be obtained when allowance is made for the right 
and left halves of the septal duplicature passing through each other, until the central 
layer in either is reached, and for the atrophy of the right half of the septal fold as 
suggested. 
4thly. When the cut ends of the common fibres found on the left ventricle, i. e. those 
to the outside of the central layer (Plate XIII. fig. 25,/), are applied to the fibres form- 
ing the two-thirds of the septum (/ which belong to this ventricle, they are ascertained 
to agree in direction, and would, if united, give rise to four complete systems of conical 
spirals (two external and two internal), these conical spiral fibres being continuous with 
each other at the apex and also at the base ; whereas the fibres of the right ventricle 
and its share of the septum, treated in the same way, although likewise continuous at 
the base and in the track for the anterior coronary artery, consist merely of spiral frag- 
ments (Plate XIV. fig. 35,// d d), and represent only a part of a more complete system 
* In this explanation I have supposed that the right ventricle and the right half of the septal fold have 
become atrophied to half their original bulk, in accordance with the law that structure and function are related 
to each other as cause and effect — the efforts required for maintaining the pulmonic circulation being probably 
about half those required for the maintenance of the systemic. The converse, however, of this explanation is 
equally true, and might he adopted with the same result as far as the comparative thickness of the ventricles 
is concerned. Thus, instead of supposing that the right ventricle and its half of the septal duplicature becomes 
atrophied, it might be assumed, in accordance with the same law, that the left ventricle and its half of the 
septal duplicature becomes hypertrophied to twice its original dimensions, the right ventricle and its share of 
the septum remaining stationary. 
