Page:Popular Science Monthly Volume 59.djvu/403

Rh In these three classes of crania the normal visual plane does not as a rule occupy the same position in relation to the horizontal plane, but varies according to the type of skull.

The relation of the normal visual plane to the type of the cranium in each of the three classes may be arrived at by direct and by indirect methods.

In the case of the living subject, the dimensions of the head may be taken and the plane of vision established in the same individual. The determination of the plane of vision in the living subject is accomplished through the aid of an instrument known as the tropometer. The relation is thus established by a direct method. The indirect method is that of ascertaining the direction of the imaginary line constitutingthe axis of the orbit in the prepared skull, the measurements of which are known. The orbits are more or less cone shaped. If the extreme apex of the cone, at which the optic nerve enters it, is taken as one point of the line of the axis, and a point where two straight lines drawn at nearly right angles with each other from certain parts of the circle of bone constituting the outer border of the orbit cross is taken as another point in the line of the axis, the line which would pass through these two points would represent the axis. This imaginary line, if projected forward and beyond the orbit, would be seen in most cases to point somewhat downward, the skull being in the primary position, and in some types of skulls it points much more downward than in other types.

It is interesting to find that the pointing of the imaginary line representing the axis of the orbit closely corresponds with the observations on the normal visual plane in the living subject.

The interest is more considerable when it is found that the form of the orbit in the different classes of skulls offers an explanation of the peculiarities in the direction of the orbital axis, as well as of the normal plane of vision.

Figs. 8, 9 and 10 represent the front views of skulls of the long, tall and broad types respectively, showing the form of the orbit corresponding to each type. It will be seen that in the long skull (Fig. 8) the roof of the orbit is much lower than that of the tall skull (Fig. 9) and that the lower border extends more downward. The orbit of the tall skull is not only placed with its opening higher, but it is more narrow from side to side. In the case of the broad skull (Fig. 10) the