196 SPECIAL PHYSIOLOGY. 



laterally, and each may be moved upon its base, in any given di- 

 rection; whilst the succeeding two joints, being hinge-joints, a certain 

 definition and greater firmness are imparted to the movements of the 

 phalanges themselves. Whilst each finger at its base, and at the last 

 joint, can be bent ordinarily only to a right angle, the intervening 

 or second phalangeal articulation can be bent to an acute angle, an 

 arrangement favored by the splitting of the extensor tendon over the 

 back of that joint; this arrangement evidently permits of a more per- 

 fect grasp. The separation of the first metacarpal bone, which sup- 

 ports the thumb, from the rest, instead of being parallel, as in the foot, 

 and the opposability of that digit to the other fingers, have already 

 been mentioned; the joint at the base of that metacarpal bone, next 

 to the wrist, instead of being a gliding joint like the others, is a modi- 

 fied ball and socket joint, capable of movement in all directions; the 

 two remaining joints of the thumb are, however, hinge-joints. 



The great distinguishing characteristic of the human hand, as com- 

 pared with the hand of the so-called Quadrumana, is, besides the 

 better proportion of the fingers, the relative length and perfection of its 

 thumb, which, when the hand is extended, reaches a little beyond the 

 middle of the first phalanx of the fore-finger; whereas, in no anthro- 

 poid ape, does it even reach beyond the base of that finger. It was 

 formerly supposed that the presence of two extensor tendons in the 

 fore and little ringers, was a peculiarity especially human ; but a 

 double extensor tendon is common in the fingers or toes of quadrupeds, 

 even though only the middle digit remains developed. 



It is necessary to add, that by the combination of length with 

 strength, and by the more refined character of the tactile endowment 

 of its broad pulps supported by its expanded nails, as well as by its 

 general mechanism and movements, the human hand is likewise dis- 

 tinguished from the hands of the anthropoid apes and the monkeys. 



Prehension and Manipulation in Animals. 



As just implied, the hand, in the anthropoid apes, is characterized by the 

 length of the fingers, and the shortness of the thumb. That of the gorilla 

 exceeds in size and power the human member ; the shortness of the free 

 portion of the fingers, owing to the extension forwards upon them of the skin 

 of the palm, together with the shortness of the thumb, are characteristics 

 which, though they may increase its mere grasping power, detract from it, in 

 comparison with the human hand, as an instrument for varied work. The 

 length of the entire limb, and that of the muscles also, are further sources of 

 strength in these animals ; the latissimus dorsi muscle is attached to the ole- 

 cranon process, or elbow, which is never the case in man (Fig. 5, 3). In the 

 anthropoid apes, the chimpanzee, the ourang, and the gibbon, the fingers are 

 proportionally longer, and the thumb shorter and weaker. In the still lower 

 monkeys, the long and taper fingers, and the diminutive, and often not oppos- 

 able, thumb, limit the use of the limb to a comparatively feeble grasp, and 

 constitute a practical mutilation of the member. In the spider-monkeys, with 

 their long grasping fingers, the thumb is wanting. In all cases, however, the 

 general formation of the upper limb, in the apes and monkeys, is similar to 

 that in man ; but the length of its various segments is such, that it is longer 

 in proportion than the lower limb, a circumstance which fits it better for 

 partially bearing the weight of the animal in locomotion, but which renders it 

 more awkward and ungainly as a dexterous prehensile, or manipulating organ. 

 It must not be overlooked here, that, in all the apes and monkeys, the foot is 



