49 
by the sensory reaction of smell (of trinitrobutyltoluol e.g. 1.106 
mgrms is distinguishable in one litre of air). In the present experi- 
ments we purposely used a rather sensitive electroscope. Our first 
care was insulation and the avoiding of sources of errors. It is, 
therefore, not out of the bounds of probabilities that for judiciously 
selected and very sensitive instruments the electrification-phenomenon 
and the sense organ will appear to vie with each other in giving 
the reaction. Apart from experimental conditions, the small quantity 
of the substance, manifesting itself by virtue of electrification is, 
as [ think, dependent on molecular weight, on volatility and on a 
lowering effect upon the surface tension. They are the very factors 
constituting the physical conditions that must be fulfilled by a sub- 
stance to act biologically as an odorous substance. 
Anatomy. — “On the determination of the position of the macula- 
planes and the planes of the semicircular canals in the cranium’. 
By Dr. H. M. pr Borrer and J. J. J. Koster. (Communi- 
cated by Prof. H. ZWAARDEMAKER). 
(Communicated in the meeting of April 28, 1916). 
§ 1. In order to be able to give an exact determination of the 
position of the macula-planes and the planes of the semicircular 
canals in the cranium, a first requirement is to connect the situation 
of these planes with fixed data taken from the cranium, which can 
easily be found again in each specimen. Absolute value cannot be 
assigned to the most exact determination, because the data taken 
from the cranium are always liable to variation; such a determination 
must therefore always be taken as an individual one. 
By comparison of found sizes with different individuals of one 
and the same species, an impression may be obtained about the 
variation and an average position can be approximated. 
The following refers to the rabbit, on whose cranium-basis we 
have fixed a line by two points, which points can easily be traced 
in each rabbit-cranium. The situation of these points moreover 
being so, as to enable us to demonstrate them accurately in series- 
sections. Pl. 1 shows the rabbit's cranium-basis seen from above. 
An imaginary line ab, connects the Incisura interecondyloidea (a) 
with a little spina (4), which regularly appears at the proximal end 
of the basi-occipitale. This line, which is the starting-point of our 
determinations, is almost conform with the line of intersection of the 
d 
Proceedings Royal Acad. Amsterdam. Vol. XIX. 
