STRUCTURE OF THE VERTEBRATES 179 



3. Abduction is drawing an entire unit of structure from its 

 median axis. When the arm is hanging at the side, and is then 

 drawn upward and outward, it is abducted from the axis of the 

 body. The thumb is abducted when it is drawn away from the 

 axis of the arm ; that is, away from the first finger. 



4. Adduction is the opposite of abduction, the function of 

 drawing a structure back into a position parallel with the axis. 



5. Circumduction consists in swinging a unit of structure so 

 that its distal end describes a circle. Holding the arm out from 

 the body and moving it so that the fingers describe a circle ful- 

 fils the conditions. 



6. Rotation, on the other hand, is turning a structure while 

 its axis remains unchanged. The head may be turned without 

 shifting the body axis; or, if the arm is held out as above 

 described, it may be rotated slightly at the joint between the 

 humerus and scapula. 



7. Pronation is limited to the lower arm and leg. Hold the 

 arm out with the palm up. Pronation turns the hand through 

 180 degrees so that the thumb is toward the median ventral line 

 of the body. 



8. Supination is the opposite of pronation, turning the thumb 

 outward from the ventral line. Supination and pronation differ 

 from rotation in that, in the former, the bones of the arm are 

 twisted upon each other, and not rotated at a joint. 



9. Elevation is raising a structure. The jaw is elevated when 

 the mouth is closed, and the ribs are elevated when the chest 

 is expanded. 



10. Depression is lowering an elevated structure. Muscular 

 contraction which depresses the lower jaw opens the mouth. 



11. Dilation is enlarging a circular opening. The reflexive 

 opening of the pupil of the eye is dilation. 



12. Constriction is closing an opening. The muscles which 

 cause constriction of openings are usually sphincter muscles, and 

 are found in the so-called valves of the digestive tract. 



Any movement of a skeletal part involves leverage. The levers 

 of the vertebrate body fall into the three classes which are rec- 

 ognized in physics. The following symbols are used in the 

 diagrams: 



