ORIGIN AND SIGNIFICANCE OF SPINES 81 



Nascent structures indicate the beginnings or initial stages 

 of organs, while vestigial structures are the remnants left 

 after the functional suppression of organs. The suppression 

 is usually caused by unfavorable conditions or by disuse, 

 which produces either a retardation of growth or a retrogres- 

 sive development. In both cases the results are similar. 

 By retardation an organ is prevented or restrained from func- 

 tional development and is therefore useless as a normal organ. 

 By retrogression an organ gradually reverts to an initial type, 

 loses its function, and becomes a vestigial structure. In 

 most instances a change of food or habit or the substitution 

 of a new and functionally higher structure causes the disuse 

 of some organ which under previous conditions was of 

 use to the animal. 



Nascent structures, or the beginnings of organs, are gen- 

 erally made up of active tissues that only require stimulus 

 and nutrition to perfect their function. On the other hand 

 suppressed or vestigial structures are composed of compara- 

 tively inert tissue, and are in consequence largely made of 

 the mechanical elements of secretion of the organism. It 

 may therefore be considered that true rudimentary or nascent 

 organs are potentially active, and suppressed structures are 

 inert. It is with the latter class, the inert, that a study of 

 spine genesis by atrophy is chiefly concerned. 



The gradual loss of function through disuse, and the con- 

 sequent loss of nutrition with the concomitant rapid decres- 

 cence of active tissues, bring about a change in the ratio of 

 active and inert structures. The progression of this process 

 naturally results in the production of a structure having a 

 maximum of inert or mechanical tissues and a minimum of 

 active constituents. Moreover, it has already been shown 

 that the axial elements are the most persistent, and therefore 

 the last to disappear; also that the peripheral appendages 

 and outgrowths of any organ first show the action of decres- 

 cence. Evidently the conditions here described are favorable 

 for the production of spines out of an organ primarily possess- 

 ing distinct active functions. The axis of an organ gives 



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