267 



In the present series no further stage of diminution in the first 

 dorsal rib is presented. A more diminutive first dorsal rib, like a 

 sixth cervical, must be of considerable raritj. 



Fig. XI. First dorsal vertebra, rudimentary first rib, left side. Second 

 dorsal rib, left side. Case C. English. Adult. Male. 



Illustrating type of rudimentary first dorsal rib in which ossification has 

 proceeded only as far as the crossing of the artery. It will be noticed that both 

 and vein cause modification of the inner border of the second rib. The lowest 

 brachial trunk makes a deep groove on the rudimentary rib. 



HI. General Conclusions. 

 Apart from the description of actual specimens, consideration of 

 the present series and of forty siieletons taken at random from the 

 Manchester collection, leads to the enunciation of certain general 

 principles. 



(1) Variations of the shape of the first rib are of very frequent 

 occurrence. 



(2) These variations are associated with variations in neighbouring 

 bones, more especially vs^ith differences in the appearance of 

 the seventh cervical vertebra. 



(3) Diminution in the impressions made by the nerve trunk and 

 the vessels on the first rib are associated with the presence 

 of cervical ribs in more or less rudimentary form. 



(4) Intensity of marking of the first rib by the structures just 

 mentioned may proceed to actual interference with the develop- 

 ment of this bone and thus specimens of rudimentary first 

 rib are formed. 



