20 C. W. M. Poynter 
Less than normal torsion, that is a rotation of the pulmonary to 
the left about the aorta through an arc of 90 degrees, may occur 
with no physiological disturbance, for the ventricular loop adapts 
itself to the slight abnormality and the structures as a whole de- 
velop along normal lines. Such a condition is easily overlooked 
and is only important as illustrating a stage of more extreme 
conditions. Robertson (1913a@) has reported four such cases and 
I have observed the condition in a foetus with rachischisis. 
More than normal torsion, that is the. pulmonary artery occu- 
pying a position in front and to the right of the aorta, is also likely 
to be overlooked and will not disturb the general development. 
The condition is perhaps most interesting as suggesting the rep- 
tilian condition. The case of Crocker (1880) is illustrative. 
All of this type of anomalies encountered cannot be explained 
by the torsion of the septum, and to account for these Lochte 
(1898) has suggested that the ventricular loop may twist nor- 
mally or the reverse independently of the other parts of the 
heart. What factors determine the direction which the ventricu- 
lar limbs will take are unknown, but it seems reasonable to sup- 
pose that they might take a reverse direction without affecting 
the rest of the heart. 
Lewis (1915) has accepted this theory and by modeling the 
heart with the ventricular loop in reverse rotation has shown that 
such development is possible. Lochte (1898) carries the theory 
farther and supposes that a left-to-right twist of the left around the 
left deviation of the interventricular septum, may occur and bring 
about a transposition of the tricuspid and bicuspid valves. 
To make a classification we may consider the truncus arteri- 
osus, aortico-pulmonary septum, and ventricular loop acting in- 
dependently. The truncus, leaving out intermediate positions, 
may assume three positions: (@) nonrotation (transposition) ; 
(b) rotation to the right (situs transversus) ; (c) rotation to the 
left (normal). The ventricular loop may rotate normally or the 
reverse, situs transversus. This will give six possible positions 
or combinations, of which one is the normal and another is the 
mirror of the normal or situs transversus. Of the four remain- 
ing combinations two may be discarded, since they are physiolog- 
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