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Nachdruck verboten. 



Numerical Variation in the Human Spine, witli a Statement 



concerning- Priority. 



By Thomas Dwight, 

 Parkman Professor of Anatomy at Harvard University. 



The purpose of the present paper is incidentally to describe some 

 new instances of numerical variation, but chiefly to discuss the present 

 state of the question and to see what effect recent investigations have 

 had on the conclusions which I offered at the close of my paper on 

 spinal variations published in 1901 ^). Finally there is a statement to 

 make concerning a question of priority. My conclusions were as follows: 



"I. Variations occur in two ways: 1) by irregular development 

 of the costal elements at and near the ends of the regions of the spine, 

 and 2) by irregular segmentation through which there are more or 

 fewer vertebrae than normal. 



II. Variations of both kinds are variations round a mean. It is 

 not impossible that some of them may be reversive ; that any are pro- 

 gressive is mere assertion. 



III. Assuming the correctness of Rosenberg's studies in onto- 

 genesis, his view may account for some of the variations, but even in 

 these cases something more is needed to explain the concomitant 

 changes. 



IV. Variation of the costal elements at one end of a region is 

 often associated with variation of an opposite nature at the other end. 

 Several regions may be involved, and the two sides may vary in- 

 dependently. 



V. Variations, which separately seem either reversive or pro- 

 gressive, generally lose that appearance when the whole spine is con- 

 sidered. 



VI. After the occurrence of the original error in development 



1) Description of the Human Spines showing numerical Variation 

 in the Warren Museum of the Harvard Medical School. Memoirs of 

 the Boston Society of Natural. Hist., Vol. 5, 1901. An abstract appeared 

 in the Anat. Anzeiger, Bd. 19, 1901. 



Anat. Adz. XXVIII. Aufsätze. 



