INTRODUCTION. 5 
termed the anterior median line; whilst the corresponding line behind is called the 
posterior median line. It is convenient to employ other terms to indicate other 
imaginary planes of section through the body. The term sagittal is therefore used 
to denote any plane which cuts through the body along a path which is parallel to 
the mesial plane (S 8’); and the term coronal or frontal is given to any vertical 
plane which passes through the body in a path which cuts the mesial plane at right 
angles (C C’). The term horizontal as applied to a plane of section requires no 
explanation. Any structure which les nearer to the mesial plane than another is 
said to be internal or mesial to it; and any structure placed further from the 
mesial plane than another is said to he external or lateral to it. Thus in Fig. 1, 
A is external to B; whilst B is internal to A. 
The terms anterior and ventral are synonymous, and are used to indicate a 
structure (D) which lies nearer to the front or ventral surface of the body than 
another structure (E) which is placed nearer to the back or dorsal surface of the 
body, and which is thus said to be posterior or dorsal. In some respects it would 
be well to discard the terms “anterior” and “posterior” in favour of “ ventral” 
and “dorsal,” seeing that the former are only applicable to man in the erect 
attitude, and cannot be applied to an animal in the prone or quadrupedal position. 
They have, however, become so deeply ingrained into the descriptive language of 
the human anatomist that it would hardly be advisable at the present moment to 
adopt this course. A similar objection may be raised to the terms superior and 
inferior, which are employed to indicate the relative levels at which two structures 
lie with reference to the upper and lower ends of the body. The equivalent terms 
of cephalic and preaxial are therefore frequently used in place of “superior,” and 
caudal and postaxial in place of “inferior.” 
The terms proximal and distal should only be applied in the description of the 
limbs. They denote relative nearness to or distance from the trunk. Thus the 
hand is distal to the forearm, whilst the upper arm or brachium is proximal to the 
forearm. 
