Terminology. 23 



concentrated. Thus the cellular material is greyish, and is hence 

 distinguished as the grey substance, while white fibrous material pro- 

 duces when concentrated an opaque white appearance similar to that 

 seen in the peripheral nerves, and is hence described as white substance. 

 In the spinal cord (Fig. 18) the grey substance is disposed as a central 

 core, the white substance as a peripheral investment. A similar relation 

 is found in the basal portion of the brain, but the characteristic pattern 

 in the cerebral hemispheres and in the cerebellum is one in which the 

 grey substance forms a peripheral, investing, or cortical layer. 



TERMINOLOGY. 



In special or descriptive anatomy it is necessary to employ an 

 extensive system of terminology in order that the various structures 

 of the body may be individually designated, classified, and referred to 

 their respective positions. The terms used for this purpose may be 

 classified into four groups, as follows: (1) general terms — those included 

 in the names of parts, but applicable in themselves to similar structures 

 (arteries, nerves, etc.) in various parts of the body; (2) specific 

 terms or names of parts ; (3) regional terms — those defining areas 

 (topographic) ; and (4) terms of orientation. 



Although few in number, the terms of orientation may be regarded as 

 the most generally useful terms of descriptive anatomy. This is because 

 they are based on very general relations of the body, and are therefore of 

 wide application. For this reason they are here selected for definition 

 to the exclusion of others of a more restricting or individualizing kind. 



In all vertebrates we may recognize a longitudinal axis, corresponding, 

 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 forwards, with respect to the 

 long axis, is anterior in comparison with any position backwards, 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 mid- 

 sagittal), 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 



