210 DR DAVID HEPBURN ON 
hardening them by walking on their knuckles and the outer 
margin of the foot. We now see why the sweat ducts open 
upon the summits of the papillary ridges; it is for the 
purpose of moistening them as the pressure of the grasping 
force becomes operative. A very dry hand is almost as ineffec- 
tive as a very wet one, and we constantly see illustrations of 
the need for artificially moistening the palm when a specially 
secure grip is desired. Now, if the sweat ducts had opened 
at the bottom of the furrows, the pressure upon the ridges 
would have occluded their mouths and prevented the escape 
of their contents; but as it is, pressure assists the outflow of 
the sweat, which moistens the resilient elastic ridge, and gives: 
the hand or foot an enormously augmented security of grasp. 
Another important application of papillary ridges is seen 
in the manner in which those which are arranged more or 
less transversely lie parallel to the long axis of a cylindrical 
object, such as a branch when it is grasped. In this position 
they prevent slipping exactly after the manner of the pattern 
upon the tyre of a bicycle. 
Certain monkeys possess prehensile tails, and whatever 
may be the value of such a tail as an aid to the sense of 
touch, it is undoubtedly relied upon as a grasping orgam 
whose non-slipping qualities are outstanding. Now the 
under or ventral side of a prehensile tail is invariably 
covered with papillary ridges, which are arranged in an 
obliquely transverse direction on each side of the mesial 
line. 
In conclusion, I think that the evidence is sufficiently 
strong to warrant the belief that, wherever papillary ridges 
occur, they are invaluable additions towards perfecting the 
security of the prehensile apparatus without in any way 
interfering with the value of the skin as an organ associated 
with touch. Since, however, prehension is most highly 
developed among anthropoids and men, among them we 
also find the fullest specialisation of the papillary ridge; but 
the very diverse functions performed by the hands and feet 
of the Bimana and the Quadrumana have resulted in an 
infinite variety of size and shape of the “pad” areas, with 
corresponding variation in the patterns which adorn them. 
Lastly, the value of these patterns as aids to the identi- 
