THE INTEGUMENTARY SYSTEM 



169 



the best possible one to prevent slipping in any direction. These finger- 

 print patterns, therefore, serve to convict men of animal ancestry, as 

 they have on occasion served to convict them of crime. 



_ FRICTION 

 ::»DIGITAL PADS. .^S^RIDGES 



^TRIRADII 



TRIRADII 



THENAR 



PAD 

 HYPOTHENAR 

 PAD HYPOTHENAR 



PAD 



A. INSECTIVORE B. MONKEY C MAN 



Fig. 126. — Friction ridge patterns in three mammals, — insectivore, monkey, and 

 man. The presence of such useless and rudimentary concentrically arranged ridges 

 in the human hand receives its only reasonable interpretation in the light of the evolution 

 theory. (Redrawn after Wilder.) 



On the sides of the fingers, the friction ridges merge into rows of wart- 

 like elevations. This has been interpreted as confirming the opinion that 

 the ridges are remnants of rows of horny scales. That the ancestors 

 of the mammals were scaly is, however, supported by more convincing 

 evidence than this. 



COMPACT 



CONNECTIVE 



TISSUE 



LOOSE 



CONNECTIVE 



TISSUE 



BASALPLATE 

 C TISSUE E. A. TISSUE q p 



Fig. 127. — A vertical section of the skin of an elasmobranch, showing five stages 

 in the development of a placoid scale. The development of a placoid scale is essentially 

 like that of a tooth. This fact taken together with the similarity of their structure sug- 

 gests that teeth may have evolved from placoid scales. (Redrawn after Schimkewitsch, 

 modified.) 



APPENDAGES OF THE INTEGUMENT 



Throughout almost the entire animal kingdom the skin tissues form 

 various calcareous, chitinous, or horny structures — shells, spines, teeth, 

 bones, scales, hair, feathers, horns — which serve for defence, support of 

 tissues, or attachment of muscles. The limy shells of molluscs and the 

 chitinous exoskeletons of arthropods serve all three purposes. 



