210 BIOLOGICAL PHYSICS 



not the whole, of life, while the depth of the furrows, 

 and, consequently, the height of the ridges, undergoes a 

 regular series of changes, according to the individual 

 experience, as to employment, and the age reached the 

 greatest height of the ridges being reached during the 

 period of most active employment, when the tactile 

 susceptibility of the sensory nervature, or the " tactus 

 eruditus," has been fully attained, and, consequently, it 

 will be found to synchronise with the greatest depth of 

 the intervening furrows except where local " tear and 

 wear " interfere. 



The reason for this will no doubt be found in the 

 greater determination of the nervine nutritive elements 

 from the central, or cerebro-spinal, neurons to the 

 constantly and acutely active peripheral nervature of 

 the digital terminal expansions and palmar surfaces. It 

 will, consequently, be found that the infant has well- 

 marked, but not conspicuously developed, ridge and 

 furrow digital details, that the youth shows more decided 

 development of these, that the adult reaches the climax of 

 development, at the period synchronising with that age 

 when the tactile sense is in continual and everyday 

 erudite use, and that when the active age limit has been 

 reached, the acutely delineated ridge and furrow markings 

 begin to show signs of levelling down, which continue to 

 grow until all that is left of them is a landmark, in the 

 form of a fragmentary and belated ridge, or furrow, 

 which at last merges into a smooth and even surface, 

 polished by the hand of time into a uniform, thin and 

 worn vestige, to contain and protect the crumbling body 

 within, and to afford a tactile surface, sufficient to maintain 

 a, more and more, restricted communication with the 

 world without, as the diminishing necessities for inter- 

 course with it cease and determine. It ought, here, to 

 be mentioned that ridge and furrow markings survive 

 longest on the surfaces of the skin of the thumb and 

 forefinger, with, perhaps, part of that of the middle finger, 

 which may be due to the manner of innervation of these 

 surfaces, but which, we are strongly of opinion, is also 

 partially due to the continuous necessity for the passage 

 of nerve impulses through these digits in particular, and 



