53 
' 
y 
hay 
a Fig. 4. b 
Construction for the determination athe angle between two planes cutting 
one another. 
between two planes. 
Making use of the same system of projection we are thus able 
to construe not only two, but all six planes of semicircular canals 
and to measure the angles «, B, y, IR, IIR, WIR, IL, UL, HIL 
(see page 2). 
As we now know the angles, which the planes of the semicircular 
canals form between them, there still remains to be determined, the 
position of the planes of the semicircular canals in the cranium. 
As has been expounded this required the knowledge of : 
1st. the angle made by a plane of a semicircular canal with the 
medium plane, 
2nd. the angle between the cranium basisline and the intersecting 
line of the planes mentioned under 1. 
The method we have been following up to now, has made known 
to us the size of the angles a, 3 and y, these are the angles formed 
by conformable planes of semicircular canals, on the right and on 
the left. In the case of absolute symmetry, the medium plane must 
be a plane that cuts the angles a, # and y into two; in other words 
the half of the values found above for the angles «, 8 and y repre- 
sents the size of the angle asked under 1"st. *) 
We know the projections of both points of definition of the 
cranium base-line, just as we likewise know the projections from 
the points a, 6 and c of the semicircular canals, which will be 
discussed more in detail. 
1) The question as to how far we can admit the existence of a perfect sym- 
metry between the right and left labyrinth will be discussed in a detailed article 
on this subject to be published later on. 
