1565 
the eve. Fick and afterwards Rvurtr have also executed some 
mensurations. Later on VoLKMANN made similar mensurations much 
more correctly with more than 30 eyes. 
VoLKMANN imagined a rectangular system of coordinates the 
axes of which pass through the center of rotation of the eye. 
The axis that connects the centers of rotation of the two eyes is 
called the w-axis, that is to say, positively from the center of rotation 
towards the temple. The axis vertically through the center of rotation 
is called the z-axis, whilst the part over the center of rotation is 
reckoned positive. The axis perpendicular to the two former, the 
sagittal axis, is cailed y-axis, the part of which behind the center 
of rotation was taken positive by VoLKMANN; in accordance with 
Zorn and Von Kries in the handbook of Hermnorrz the part of the 
y-axis before the center of rotation will however here be reckoned 
positive. 
The position of the head is by no means insignificant for the 
mensurations of the location of the points of origin and insertion 
of the eye-museles according to this system of coordinates ; the head 
is supposed to be kept erect. 
VOLKMANN admitted as point of insertion of the muscles of the 
eye the center of the line of insertion. If now we call the coor- 
dinates for the insertion of the m. obliquus superior #;, y; and 2; 
and for the trochlea «,, y, and z, then the result of the measurements 
made by VoLKMANN produced the following averages: 
n= mm yi = —4.41 mm. 2— O LD annonr 
= —15.27 mm. y = 8.24 mm. Zo = 12.25 mm. 
For his calculations VoLKMANN admitted, that the normal eye 
corresponds with a globe, the radius of which amounts to 12.25 mm. 
whilst the point of rotation would lie 1.29 mm. behind the center 
of this globe, as has been determined by Dorpers and Doyer. It is 
necessary, that the point of rotation has a constant location not 
only in the eye but also in the orbita, as we determine both the 
place of insertion and the place of the trochlea with regard to the 
center of rotation. Most likely there exists neither in the orbita, 
nor in the balbus oeuli a real constant center of rotation. The 
investigations of HwrmHortz, Donprrs, Mürrer, VOrLKMANN, Wo1now, 
Beruin and others have taught us however, that we are certainly 
not far from the truth, if we admit a constant point of rotation 
for the normal eye, so that consequently the region called by Hering 
the interaxial space of the bulbus, is also very small. Therefore we 
shall not make great errors, if we continue to make use of the 
data supplied by VoLKMANN in this respect. 
88* 
