4 HUMAN ANATOMY. 



vertebrates. Superior and inferior, upper and lower, as indicating relations towards 

 or away from the head-end of the body, are, probably, too convenient and useful as 

 expressing the peculiar relations in man to be readily relinquished, although when 

 directly applied to animals possessing a horizontal body-axis they refer entirely to 

 relations wath the plane of support, the additional terms cephalic and caudal being 

 necessary to indicate relations with the head- and tail-pole. Likewise, "anterior" 

 and "posterior," as referring respectively to the front and back surfaces of ;;he 

 human body, are more logically described as ventral and dorsal, with the advantage 

 that these terms are directly applicable to all vertebrates. ' ' Outer' ' and ' ' inner, ' ' as 

 expressing relations with the sagittal plane, are now largely replaced by the more 

 desirable terms lateral and mesial respectively, external and internal being reserved 

 to indicate relations of depth. Cephalic and caudal, central and peripheral, prox- 

 imal and distal, are all terms which have found extensive use in human anatomy. 



Preaxial and postaxial, in addition to their general and obvious significance 

 with reference to axes in common, have acquired a specific meaning with regard to 

 the limbs, the appreciation of which requires consideration of the primary relations 

 observed in the embryo. In the earliest stage the limbs appear as flattened buds 

 which project from the side of the trunk and present a dorsal and ventral surface ; 

 subsequently the limbs become folded against the body, the free ends being directed 

 ventrally, while one border looks headward, the other tailward. If an axis corre- 

 sponding to the transverse plane of the body be drawn through the length of the 

 extremities, each limb will be divided into two regions, one of which lies in front of 

 the axis, and is, therefore, preaxial, the other behind, or postaxial. On reference to 

 Fig. 3 it is obvious that the preaxial border or surface of each limb is primarily 

 directed towards the cephalic or head-end of the animal, and, conversely, that the 

 postaxial faces the caudal or tail-end. These fundamental relations are of great im- 

 portance in comparing the skeleton of the upper and lower extremities with a view 

 of determining the morphological correspondence of the several component bones, 

 since the primary relations become masked in consequence of the secondary dis- 

 placements which the limbs undergo during their development. 



The terms homologue and analogue call for a passing notice, since an exact 

 understanding of their significance is important. Structures or parts are homologous 

 when they possess identical morphological values founded on a common origin ; thus, 

 the arm of a man, the front leg of a dog, and the wing of a bat are homologues, because 

 each represents the fore-limb of a vertebrate, although they differ in individual func- 

 tion. On the other hand, the wing of a bat and that of a butterfly are analogous, 

 since they are structures of functional similarity, although of wide morphological 

 diversity. Homologue, therefore, implies structural identity, analogue implies 

 functional similarity. Parts are said to be honiotypes when they are serial homo- 

 logues ; thus, the humerus and the femur are homotypes, being corresponding 

 structures repeated in the same animal. Where parts possess both morphological 

 and functional identity, as the wing of a bird and of a bat, they are analogous as well 

 as homologous. 



