472 HISTORY OF THE HUMAN BODY 



cases concerns the epidermis alone and consists of a concentric 

 arrangement of a small group of cells. If this homology be 

 a true one we must also consider the amphibians as the direct 

 ancestors of mammals, since the lateral line organs do not 

 occur in reptiles. In this comparison the original sense-organ 

 is, of course, the equivalent of the convex hair papilla, which 

 lies at the root, covering the corium papilla, and from which 

 proliferate the cornified cells of the hair shaft. Something 

 analogous to this exists in the so-called " pearl-organs," horny 

 bodies which develop from certain of the lateral line organs 

 in some fishes, showing that there is present in these organs 

 a tendency to produce cornified structures. 



Again, the sensitiveness of the hair root, and its abundant 

 nerve supply, especially in cases like that of the vibrisscz (whis- 

 kers) of the upper lip in many mammals, speaks in favor of 

 such a derivation. The great multiplicity of the hairs, con- 

 sidering that each represents an original sense-organ, and also 

 their almost universal distribution, is paralleled by the adap- 

 tive multiplication of other parts, such as the mammae in some 

 forms or the vertebrae in elongated animals. On the other 

 hand, the test of nerve supply fails to even suggest this hypoth- 

 esis, since in the earliest terrestrial vertebrates the extensive 

 system of superficial nerves associated with the lateral line 

 organs becomes entirely lost (unless the Acusticus may be 

 looked upon as derived from it) ; furthermore, the close asso- 

 ciation between hairs, scales, and integumental glands turns 

 the argument in a totally different direction. (Cf. Chap. IV.) 



The third possible derivative of the lateral line system, the 

 inner ear, does not come into the same category as the taste- 

 buds and the hair, since if it came from this system at all, it 

 must have separated from it very early, and thus could not in 

 any case be considered a survival of the system as it exists in 

 fishes and amphibians. This theory receives its strongest sup- 

 port from the developmental origin of the Eighth nerve, which 

 has been clearly proven to segment off from that part of the 

 Seventh which supplies the lateral line organs, certainly a 

 strong argument, since, if the Eighth nerve were once an ele- 



