CHAPTER I. 



TERMS DENOTING POSITION AND DIRECTION OF PARTS. 



Need of a technical nomenclature.— It has been found 

 that the use of the terms upper, lower, inner, outer, before, 

 behind, anterior, posterior, and similar expressions in the 

 technical descriptions of animals or their parts frequently 

 leads to ambiguity. A great part of the confusion doubtless 

 arises from the fact that the natural position of man differs 

 from that of the lower animals in being erect. Thus, for ex- 

 ample, when applied to men, before means in the direction 

 indicated by a line drawn from the center of the body to the 

 ventral surface ; in the lower animals it means in the direc- 

 tion indicated by a line drawn from the center of the body to 

 the head. The same difficulty attends the use of the term 

 anterior ; and of the opposite of these terms, behind and 

 posterior. 



Another source of confusion in the use of this class of 

 terms is the fact that they are very commonly applied with 

 reference to the plane of the horizon. Thus above means 

 towards the zenith ; below^ towards the nadir ; and before and 

 /^^/^/Vz^ indicate directions parallel to the plane of the horizon. 

 Consequently whenever the position of an object is changed 

 the terms denoting the relation of its parts must be changed. 



In order to avoid these difficulties a special set of terms 

 for designating the position and direction of the parts of 

 animals has been adopted by many writers ; and it is the 

 object of this chapter to define such of these terms as are 

 used in this book. 



