340 O. E. Beecher — Origin and Significance of Spines. 



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 

 produce either a retardation of growth or a retrogressive de- 

 velopment. In both cases, the results are similar. By retarda- 

 tion, an organ is prevented or restrained from functional devel- 

 opment 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. Jn 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 gener- 

 ally made up of active tissues that only require stimulus and 

 nutrition to perfect their function. On the other hand, sup- 

 pressed or vestigial structures are composed of comparatively 

 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, brings 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 out- 

 growths of any organ first show the action of decrescence. 

 Evidently, the conditions here described are favorable for the 

 production of spines out of an organ primarily possessing dis- 

 tinct active functions. The axis of an organ gives the neces- 

 sary form, and the hard tissue the structure, so that the whole 

 will conform to the definition of a spine given early in this 

 paper; namely, a stiff, sharp-pointed process. 



The restraint of the environment was found to be one cause 

 for decrescence of organs. Another, which is properly the 

 subject matter of the present section, is disuse; and lastly, it 

 will be seen how^ the deficiency of growth force may bring a 

 similar suppression of structures. 



There is considerable difficulty in selecting particular exam- 

 ples which will conform clearly to the strict requirements of 

 these three categories. In a certain sense, some of the exam- 



