130 0. E. Beecher — Origin and Significance of Spines. 



A. External Stimuli. 



Under external stimuli are included all the forces of the 

 environment (chemical, physical, organic, and inorganic) which, 

 through their impact or influence on an organism, produce a 

 consonant favorable change or disturbance. In general, it will 

 be seen that the number of impressions and their power will 

 depend largely upon the position and character of the surface 

 upon which they impinge. The more exposed the position, the 

 greater will be their strength and number, and if these stimuli 

 or impressions are intermittent, and not so violent as to pro- 

 duce waste and rupture, growth will ensue. Under ordinary 

 conditions, exposed parts will naturally be the first to receive 

 sufficient stimulus to produce growth, and there will be nor- 

 mally a direct correlation between growth and stimulus. 

 In a simple diagrammatic form, this would be expressed 

 by a series of lines, the first representing a plane surface. 

 Then, owing to the impossibility of maintaining a uniformly 

 intermittent stimulus or a uniform response, some point or 

 spot on this surface would grow in excess of the others. 

 This difference would be augmented by the more favorable 

 position of the spot to receive stimuli, further growth 

 would take place, the growth force decreasing with the in- 

 crease of distance, and the final action of these forces, stim- 

 ulus and growth, would be to produce a pointed elevation. 

 Such structures or outgrowths, especially when made of 

 hard rigid tissue, would be termed spines under the general 

 definition. The spine may be viewed as an attached organ- 

 ism, and its conical habit of growth would then conform 

 to the law of radial symmetry, as determined by the physio- 

 logical reaction from equal radial exposure to the environ- 

 ment. That all the irregularities of contour in all organisms 

 have not developed into pointed processes or spines is not, 

 therefore, the fault of the simple reciprocity between growth 

 and external stimuli. This kind of development, however, re- 

 quires a direct and immediate responsive external growth to 

 the exciting force, which from various causes is frequently 

 absent. Obviously, stimuli which result simply in motion or 

 equivalent internal adjustments can have no effect toward 

 spine production, so that only the results of such stimuli as 

 bring about some accompaniment of superficial growth will 

 be considered. 



With the exception of perfectly spherical, freely moving 

 forms, all organisms have certain parts which are more ex- 

 posed to the forces of the environment than others, and from 

 the principles already enunciated, such exposed parts under 

 normal conditions will grow. This growth in the direction of 



