— 4 — 



rection indicated by a line drawn from the center of the ani- 

 mal to the head, but at an infinite distance in that direction. 

 In other words, these terms must be used in a way analogous 

 to that in which we use right and left. 



Examples.— Take a figure of a Dragon-fly with its wings extended 

 as when at rest. Draw a line from the distal extremity of one of the 

 wings to the head. Although this line extends directly towards the 

 head it does not extend cephalad ; but more or less nearly mesad.* 

 A line extending cephalad from the distal extremity of a wing (or 

 from any other part) is parallel to the cephalo- caudal axis of the 

 body. 



Differences between the technical and popular uses 

 of cephalic and caudal. — It has just been shown that in 

 the use of these terms it is not the head and tail to which the 

 position and direction of parts are referred, but to two of 

 the cardinal directions which are at right angles to right and 

 left. Thus when an insect extends its antennae cephalad, 

 the most cephalic segments of these organs are those farthest 

 from the head. For the same reason we can speak of the 

 caudal part of the head or of the cephalic portion of the 

 tail. It will be seen that this does not accord with the 

 popular uses of these terms (as defined in the dictionaries) 

 according to which no part of the body is cephalic except 

 the head ; and of the different parts of the head one is just 

 as much cephalic as another. 



Oblique lines. — The position or direction of a part 

 towards a point between two of the cardinal points, or 

 toward a point between one of the cardinal points and a line 

 connecting two other cardinal pojnts can be designated by a 

 compound term. 



Examples. — A part which extends in a direction between those 

 directions which are indicated by dextrad and caudad is said to extend 

 dextro-caudad. A part which extends in a direction between those 

 directions which are indicated by dorsad and dextro-caudad is said to 

 extend dorso-dextro-caudad. This last direction would be indicated 

 in popular terms by saying : Towards the back and to the right and 

 towards the tail. 



* Mesad is defined on a later page. 



