It should be carefully noted that cephalad does not neces- 

 sarily mean towards the head but headward ; that is, to- 

 wards a point which is in a direction indicated by a line 

 drawn from the center of the animal 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. 



Example. — 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 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 cephal- 

 ic as another. 



Oblique lines. — The position or direction of a part 

 towards a point between two of the cardinal points can be 

 designated by a compound term. 



Example. — A part which extends in a direction between those direc- 

 tions which are indicated by dextrad and caudad is said to extend dextro- 

 caudad. 



Meson, mesal, and mesad. — Frequently the position 

 or direction of a part is referred to an imaginary plane di- 



* Mesad is defined later. 



