34 



INTRODUCTION*. 



As already indicated, the position of the meson is fixed ; that of the 

 other two planes is purely conventional and for convenience in a given case. 



Ventral aspect. 



FIG. 5. DIAGRAM OP THE THREE BODY-PLANES, WITH THE 

 VARIOUS LINES OF DIRECTION. 



64. Designation of Position and Direction. By the use of terms 

 derived from meson, and from the names of the general regions or aspects 

 of the body, it is possible to designate positions and lines of direction with 

 considerable precision. 



There are two general cases : 



1. Where the points to be located are on a line which itself lies in one of 

 the three body planes, or in a plane parallel to one of them. 



2. Where the points to be located are on a line which is not in either of 

 these planes, but connects some two of them like the diagonal of a solid ; 

 e. g., the points H, K, at the ends of the line HK. 



Under case 1, the lines may be either direct or oblique. 



65. Direct lines may be perpendicular to the meson, as the line SFI, 

 or any other line passing directly from one side of the body to the other. 

 Such a line would be the line connecting the eyes, or the heads of two ribs. 



Direct lines may lie either in the meson itself, or in an imaginary plane 

 parallel thereto and either parallel or perpendicular to the longitudinal 

 dorso-sinistral plane ; e. #., ABC or MHL. Such lines pass directly from 

 the dorsal to the ventral aspect of the body, or from the caudal to the 

 cephalic aspect. 



All direct lines may be also characterized as coinciding with the inter- 

 section of two of the three body-planes, or of planes parallel thereto. 



