THE INTEGUMENT AND THE EXOSKELETON 91 



and plantar surfaces in Man and the other higher primates 

 developed primarily in forms in which the entire surface was 

 used for contact, it may be assumed that they would have taken 

 some simple form, designed with reference to the area as a 

 whole; since, however, they have passed through the longer 

 and more complex history caused by the introduction and 

 secondary reduction of various pads, they have preserved the 

 indications of the former relief by an arrangement otherwise 

 without cause or meaning. This may be seen by a compari- 

 son of the lower surface of the paw in some animal in which 

 the pad system is in full function with that of one in which 

 the inequalities of the surface have become secondarily re- 

 duced. (Fig. 24.) The one is an actual relief, the other a 

 flat sketch ; the one possesses raised pads surrounded by folds 

 of skin which diverge in three directions from points known as 

 triradii, the other indicates the former location of the pads by 

 whorls and other patterns, and that of the folds by the arms 

 of embracing triradii. Thus in the field-mouse (Fig. 24, a) 

 there are present four interdigital pads, the first situated im- 

 mediately below the interval between thumb and index, the 

 second below the interval between the latter and digit III, 

 and so on. Each of these is inclosed by folds of skin which 

 diverge in three directions from points known as triradii, and 

 there are three triradii about each pad except the third, which 

 possesses a fourth one, located between digits III and IV. 

 Below these lie the thenar and hypothenar pads, the folds of 

 which are often well marked, though not especially so in this 

 case. The apical pads at the ends of the digits also possess 

 folds, not well shown in the figure, with two triradii, one upon 

 each side. Turning now to the paws of Macacus, a small 

 monkey (Fig. 24, b), in which the relief has been reduced to 

 a flat surface, each of the above features (except the thenar in 

 this especial case) is expressed by the configuration of the 

 ridges, as indicated in the figure. The ridges essential in 

 marking the palm are represented by solid lines, although in 

 reality not different from the rest. Each pad is represented 

 by a figure or i, of which the four interdigital are the 



