90 HISTORY OF THE HUMAN BODY 



directly traceable to them. In their simplest form the scales 

 or scale rudiments are in the form of rounded, wart-like epi- 

 dermic organs, which cover the entire surface in Ornitho- 

 rhynchus and large portions of it in marsupials, insectivores 

 and lemurs. They possess a more or less imbricated arrange- 

 ment, and their identity with scales is shown, not merely by 

 their structure and development, but by a comparison with the 

 scaled dorsal surface of the paw in such cases as that of the 

 shrew or the star-nosed mole, where the transitions from one 

 form to the other may be seen along the edges of the paw. 

 This primitive condition is modified in most cases by the pres- 

 ence of characteristic mammalian organs, the pads,- which 

 are used as contact surfaces, and are typically eleven in num- 

 ber, five for the tips of the digits, four for the distal margin 

 of palm or sole, below the interdigital intervals, and two near 

 the wrist or ankle. Upon these the scale rudiments become 

 arranged in rows, and by their fusion form friction ridges, 

 so called from their use, which is to prevent slipping, like the 

 parallel ridges seen on the handles of certain steel instruments. 

 These friction ridges are always arranged at right angles to 

 the direction in which there is the greatest tendency to slip, 

 that is, directly across the pads in walking forms, but arranged 

 in concentric circles about the highest part of the pad in the 

 arboreal lemurs and monkeys where slipping in all directions 

 is equally to be expected. Owing to the general principles that 

 the separate scale rudiments form friction ridges on the ac- 

 tual contact surfaces only, it follows that when the pads re- 

 main high their surfaces alone are ridged, while the depressed 

 areas are covered with separate units, but when, as in lemurs 

 and monkeys, there is a progressive tendency to utilise the 

 entire surface for contact, the ridged areas spread in exact 

 correspondence with the acquirement of contact surface, un- 

 til, in the higher primates, the entire ventral surface of the 

 paws becomes covered with ridges, leaving separate scale rudi- 

 ments only along the boundaries, where this modified skin 

 meets that of the dorsal surface. 



Had the friction ridges, which completely cover the palmar 



