678 



COMPARATIVE ANATOMY 



will appear among the scales in a very definite relationship, namely, a 

 group of three hairs (one medial and two lateral) will project beneath 

 the margin of each. The median hair is the longer and stouter. In 

 addition to this, there are similar arrangements of hairs in groups of 

 three even upon areas not definitely associated with scales. The hairs, 

 however, are arranged in an imbricated series like scales. Even where 

 the hair is very thick, and forms a heavy fur, this arrangement can often 

 be made out. 



As scales in their simplest form are tiny elevations, the pads on 

 mammalian feet are often used to illustrate the arrangement and transi- 

 tion of scales in different mammalian groups. 



These pads are usually eleven in number, five for the tips of the 

 digits, four for the distal margins of palm and sole beneath the inter- 

 digital intervals, and two for the wrist or ankle. 



The scale, rudiments are arranged in rows upon these pads. They fuse 

 to form "friction ridges," so-called because they prevent the animal from 

 slipping (Fig. 402). These friction ridges are always arranged at right 



angles to the direction in which 

 there is considerable tendency 

 to slip. In the arboreal types 

 of mammals, such as the lemurs 

 and monkeys, the scale rudi- 

 ments are arranged in concen- 

 tric circles as in such animals 

 there is a tendency to slip in 

 any and all directions. The 

 ridges form only on the actual 

 contact surfaces. 



While structure always de- 

 termines function, yet in integ- 

 umental studies we have found 

 that function very decidedly 

 modifies the various structures, 

 and later, we shall see that such 

 modification is not confined to 

 integument alone. 



Now, to be truly scientific, 

 means to retain an open mind 

 to all truth wherever and whenever found. But our prejudices and 

 wishes all too often influence us as readily toward a too conservative as 

 toward a too radical point of view. We must face the facts as they are, 

 pleasant or unpleasant, but we must not forget that many different in- 

 terpretations can be drawn from the self-same facts. An example of 

 this is brought home at this very point. 



There is no question about the facts so far presented, which anyone 



A. B. 



Fig. 402. 



Ventral view of the palm of the hand of an 

 insectivore and of a primate to show correspondence 

 between relief and arrangement of friction ridges. 

 A, Crocidura coerulea (shrew-mouse). Forepaw 

 showing walking-pads enclosed by triangular folds 

 of skin. B, Macacus sp? (Old World monkey), Hand, 

 covered by friction ridges, the arrangement of which 

 corresponds to the relief of A. The pads are rep- 

 resented by concentric circles, and the triangular 

 folds by triradii. These latter features are here 

 designated by heavy lines, although in the animal 

 they are no more conspicuous than the others. (From 

 Wilder after Miss Whipple.) 



