IN THE LIMBS OF MAMMALIA. 65 



limbs is evident ; the arm or humerus and the thigh or femur, the fore-arm and the leg, 

 the hand and the foot, the fingers and the toes are easily seen to occupy similar positions 

 in the limbs to which they belong ; but the law of longitudinality is further carried out 

 among these segments according to what may be called corollaries thereof. 



1st. Two corresponding segments in the fore and hind limbs point, and are flexed or ex- 

 tended, in absolutely opposite though relatively similar directions. 



2d. Two contiguous segments of the same limb also point, and are flexed or extended in 

 opposite directions, so that the flexor muscles of one segment lie on the same side of the limb 

 with the extensors of the segment next above or below, and vke versa. 



All this is easily seen in the mounted skeleton of any quadruped ; the scapula and ilium 

 diverging, the humerus and femur converging, the fore-arm and leg again diverging, the 

 foot pointing forward, and the toes, since their flexion is in the direction opposite to that of 

 the foot at the ankle, really pointing backward ; but their antitypes, the hand and fingers, 

 seem to point also in the same instead of the opposite direction, so that two and even three 

 sets of muscles, which by their contraction shorten the arm, lie upon one and the same side 

 of the limb. 



To understand this apparent anomaly, it must first be remembered that the entire limbs 

 are teleological, and that the influence of morphology diminishes as we recede from the 

 morphological centre toward the distal extremities ; moreover, the functions of the hands 

 are various to the highest degree, and they, as the special agents of the brain, may be sup- 

 posed to partake somewhat of its independence of morphological restraints. 



Embryology throws light on this point. In the early foetal periods, the two bones of the 

 fore-arm are parallel, and this alone is sufficient indication of their morphological relation 

 in the course of development the hand is gradually pronated, so that the lower end of the 

 radius, the outer bone, crosses the ulna, and so becomes internal, causing the palm to face 

 downward and backward, instead of, downward and forward. In this relation, more or less 

 firmly connected, the two bones remain in quadrupeds ; for they, not being stationary geo- 

 metrical figures, but organized living creatures intended to move from place to place, must 

 be able to strike the earth alike with both pairs of limbs in order to propel the body in the 

 opposite direction ; but in monkeys and in man, where the anterior extremities are not 

 merely for progression but for executing the higher mandates of the will, the radio-ulnar 

 articulations remain free, and the parts may be restored to their normal condition by supi- 

 nating the fore-arm, the palm still facing downward, but now also forward instead of back- 

 ward ; the fingers flex forward and the toes backward, although made to act as continua- 

 tions of the larger segments, hand and foot. 



Morphologically, the flexion of the hand at the wrist must be in a direction opposite to 

 the flexion of the fore-arm at the elbow, and therefore the muscles which raise the so-called 

 back of the hand toward the back of the fore-arm are really the flexors of the former seg- 

 ment, and correspond to the muscles which elevate the dorsum of the foot toward the 

 front of the leg ; and, per contra, the muscles which bend the palm of the hand toward 

 the inside of the fore-arm, are really extensors, and correspond with those which in the 

 posterior limb act upon the foot through the tendo Achillis ; that is, the muscles now 

 called extensorcs carpi radialis and ulnaris, are morphologically flexors, and their antagonists, 

 now called flexores carpi radialis and ulnaris, are extensors, and will be so designated in this 

 paper. 



There are two apparent objections to the above interpretation of the antitypical rela- 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. 17 



