Page:Popular Science Monthly Volume 59.djvu/401

Rh By the term 'normal plane of vision' as here employed, it is intended to express the direction which the lines of sight would assume, the head being in the erect, or more technically in what is known as the primary, position, in case no muscular effort should be made to change the eyes from an entirely passive adjustment.

Under these conditions the line of sight of each of the two eyes lies in an imaginary plane which may be coincident with, or somewhat higher than, or somewhat lower than, the plane of the horizon.

The normal plane of vision differs essentially from the primary plane of regard of Helmholtz and other writers, for this plane of regard refers to the plane formed by the two visual lines when these lines are directed toward the horizon, the head being in the primary position. The plane of regard is therefore unalterable. It was my own privilege to show that the normal plane of vision not only varies in different individuals but that, as a general rule, this variation is associated with and controlled by certain cranial characteristics which will presently engage our attention.

This leads us to the consideration of certain types of the human cranium as they are recognized by craniologists.

While the form of the head of an individual may not be so clearly of one or another type that it must be classified as belonging to a certain group, in general, heads are grouped into three great classes or types, and these classes or types are again divided into subtypes. In this connection the subtypes need not be taken into consideration, but some knowledge of the main types is essential.

Craniologists, then, classify crania as long, broad and medium. Medium skulls, in order to avoid misconception, will be here designated as tall skulls, since the term medium does' not, in this relation, refer to capacity, but to certain special measurements, and the accepted term might be misleading to those not well versed in the subject.

The basis for the classification consists of the proportion which the longest diameter from before backwards bears to the longest transverse diameter. If the transverse diameter is $75/100$ that of the longer diameter or less than $75/100$ the head is said to be in the class of long heads; but if the transverse diameter equals or exceeds $85/100$ the length of the skull, it is a broad skull. Medium skulls, or, as we are now to call them, tall skulls, are those in which the transverse diameter is between $75/100$ and $85/100$ and, as might be supposed, the measurement from the base of the skull to its summit, while it may not of itself be greater in an individual case than that of one of the long or one of the broad type nor even so great, is greater in proportion to the other measurements.