1887.] NATURAL SCIENCES OF PHILADELPHIA. 61 



abduction, adduction, pronation and supination imperfectly express 

 many animal movements. Thus no adequate word exists for the 

 movement of the hand upward and downward when the forearm is 

 held in semipronation. Such a motion is of importance in describ- 

 ing the wing of the bird and the bat. The term Planation is 

 here thought to be convenient since it expresses the fact that the 

 movement of the hand is in the plane of the flexor surface of the 

 forearm. Planation includes both j^ronation and supination. 

 Contraplanation as easily indicates a movement at an angle to the 

 plane of the flexor surface of the forearm. It embraces flexion and 

 extension and is not es2:)ecially demanded, except that a term which 

 includes both flexion extension as well as abduction, as used in the 

 sense of abduction of the thumb, may be acceptable. 



In like manner it has been found useful to employ terms for the 

 different orders of the foot-falls. 



Synehiry indicates that the right and left foot of a single pair 

 act together. Thus in the gallop the horse moves all the feet 

 synchirally. The movement of the lower limbs in man is also 

 synchiral. The feet may act alternately or together. 



Asyitchh'y would naturally embrace the movements in which 

 the feet act in combinations of hind and forefeet. But since these 

 movements are varied and important, it has been thought desirable 

 to substitute a positive term, and the word heterochiry ^ is j)ro- 

 posed. The walk, the trot and the rack are heterochiral, since 

 the fore foot is follow^ed, not by its fellow, but by a hind foot. 

 When the hind foot alternates with the fore foot of the same side 

 lateral heterochiry occurs. When with the fore foot of one side 

 the hind foot of the opposite side alternates, diagonal heterochiry 

 takes place. 



In connection with the terms flexion and extension the following 

 will be used : The movement of a limb against the medium in 

 which the animal is moving constitutes the "stroke." The 

 movement in preparation of the stroke constitutes the "recover." 

 In the description of the " hand over hand " movements of the 

 sloth and the monkey, the word " flexion " has no place ; yet the 

 " recover " is used in as exact a sense as in the movements of any 

 other animal. 



1 While assuming the responsibility for this word the writer desire at the same 

 time to say that it was suggested to him in a conversation with his friend Prof- 

 T. N. Gill. 



