Terminology. 41 



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 caaal; 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. 



