TERMINOLOGY. 4I 
the lowermost surface as ventral, the sides of the body as lateral. 
Any position forward, with respect to the long axis, is anterior 
in comparison with any position backward, which is posterior. 
In relation to the long axis it is convenient to recognize a 
median vertical plane of section, which is one dividing the body 
into right and left halves; also transversal planes, which are 
planes situated at right angles to the median plane and to the long 
axis, and sagittal planes. The latter are vertical planes parallel 
to, and also including (as midsagittal), the median vertical plane. 
The median vertical plane is the plane of bilateral symmetry, 
each half of the body as thus defined being in a general way the 
reverse counterpart of the other. Structures situated in part in the 
median plane are unpaired, and are described as median, while 
structures situated wholly outside of the plane are paired, right 
and left, or dextral and sinistral. In relation to the median plane 
and to the sides of the body, structures are described as medial 
when nearer the former, and as lateral when nearer the sides of the 
body. The term intermediate is applied especially to a position 
between medial and lateral, but this restriction is perhaps not 
justifiable. 
In considering the extent of bilateral symmetry, it is necessary 
to bear in mind that, although a fundamental feature in verte- 
brates, it is not perfectly retained in the adult condition: Symmetry 
is destroyed by the migration of an unpaired structure from a 
median to a lateral position, as is seen, for example, in the case of the 
abdominal portion of the alimentary canal; or, again, by the reduc- 
tion or disappearance of structures belonging to one side of the 
body, as, for example, in the case of the mammalian aortic arch. 
Referring to centre and circumference, either in the body as a 
whole, or in particular parts, the terms deep and superficial, 
central and peripheral, or internal and external may be 
applied. It may be observed, however, that the terms internal and 
external are sometimes used in the sense of medial and lateral, both 
in descriptive language and in the names of parts. 
In comparison with the terms medial (medialis) and median 
(medianus) the term middle (medius) may be used to designate 
the position of a structure lying between two others, the latter being 
otherwise designated, for example, as anterior and posterior, or one 
_ in the median plane. 
