TERMINOLOGY 35 



anatomical description to be intelligible. For these reasons they 

 are selected for definition here to the exclusion of others of a more 

 restricting or individualizing kind. 



In all vertebrates we may recognize a longitudinal axis, corre- 

 sponding, in general, to the line formed by the vertebral column. 

 In the usual or prone position of the body this axis is horizontal. 

 The uppermost surface is then described as dorsal, 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, which is one dividing the body into right 

 and left halves, and transverse, coronal, and sagittal planes. 

 Transverse planes are situated at right angles to the longitudinal 

 axis. Coronal planes are longitudinal and horizontal, at right 

 angles to the median vertical plane. Sagittal planes are longi- 

 tudinal and vertical, parallel to and including (as midsagittal) the 

 median vertical plane. 



The median vertical plane is the centre of bilateral symmetry, 

 each half of the body, as divided by it, being in a general way the 

 reverse counterpart of the other. Structures or situations partly 

 in the median plane are unpaired, and are described as median, 

 while positions 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 

 reduction 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 



