INTEODUCTORY 39 



(1) The trunk presents right and left .sides; a back, or dorsal side; 

 a belly, or ventral side ; a head or cephalic end, and a tail or caudal 

 end. The usual position of the vertebrate body is with the belly 

 toward the ground ; hence the ventral side is also called the lower 

 or inferior side, the dorsal side is the upper or superior side, the 

 cephalic end is the anterior end, and the caudal end is the posterior 

 end. Moreover, the body as a whole is divided into right and left 

 halves by a median plane. 



Bones, like other organs, are situated within the body, and hence, 

 in describing their various sides and their relations to one another 

 and to other organs, terms of direction are usually employed. For ex- 

 ample, the side or end of a bone near the head is called the anterior 

 side or end, or, if one bone lies nearer the head end than another bone, 

 it is said to be anterior to it. The upper or superior end of a bone is 

 the end near the upper side or back of the body ; one bone nearer 

 the back than another is said to be above or superior to the other. 

 The end or side of a bone near the median plane is called the inner 

 end or side ; of two bones the one nearer the median plane is said to 

 lie internal to the other. Moreover, lines drawn at right angles to 

 the median plane are transverse lines ; lines in or parallel with the 

 median plane are longitudinal lines, and lines connecting the back and 

 the belly are vertical lines. These terms, which are extrinsic, based 

 upon the animal's posture, would be sufficiently accurate and universal 

 if every animal walked on four limbs, but some animals among them, 

 of course, man walk on the posterior limbs only, and have assumed, 

 in place of a horizontal posture, an upright one. Hence the terms we 

 have given do not mean the same things when we speak of the human 

 body ; ventral means anterior, the head end is not the anterior end, 

 but the superior end, and the dorsal side is not the superior side, but 

 the posterior side. 



We need, therefore, terms which can be applied to any animal 

 without regard to its posture. Such terms are called intrinsic terms ; 

 they are based upon the relation of one part of the body to another 

 part. 



The head end of a bone,' therefore, is intrinsically its cephalic end, 

 and one bone lies on the cephalic side of another if nearer the head. 

 The tail end of a bone is the caudal end. The end near the belly is 

 the ventral end (Fig. 12), or one bone, or feature on a bone, is ventral 



