268 



(5) The variations of more frequent occurrence are those which 

 represent comparatively slight modification of the normal 

 condition. 



They decrease in frequency the more they approximate 

 to the extreme forms, i. e., the presence of a complete seventh 

 cervical rib or the total absence of the first dorsal rib. 



(6) The variation of nerve and vessel influences does not occur 

 pari passu. 



(7) Vascular influence is more apparent as a rule on the left side 

 and especially is this the case vs^ith arterial influence. 



(8) Nerve influence occurs in more equal frequency and extent 

 on both sides and is complicated by the arrangement of the 

 cords in the brachial plexus. 



This statement requires furthel^ evidence to be produced 

 in a later paper. 



(9) Effect of nerve or vessel influence on skeletal tissue as illus- 

 trated by the ribs at the thoracic inlet, may occur in any of 

 the three following modifications: 



(i) Fusion of the two highest ribs on either side may 

 be induced in part of their extent and thus the 

 uppermost appears rudimentary. 



(ii) A portion of the highest rib may be hindered in its 

 evolution toward bony tissue and thus may be left 

 as connective tissue in ligamentous form, 

 (iii) A portion of the rib may be entirely inhibited in 

 development and thus the rudimentary remnant of 

 the bone may at one or other extremity lie free in 

 surrounding tissues. 



(10) AVhile modifications (i) and (ii) are more frequently associated 

 with vessel influence, (iii) is more characteristic of a nerve 

 effect. 



(11) The observations in the present research on rudimentary ribs 

 are in accord with, and confirmatory of, those set forward in 

 a previous paper, which dealt with the relations of the normal 

 thoracic operculum. 



(12) The deficiency in the bony tissue of the highest rib or the 

 loss of its individuality through its fusion with the second 

 member of the series most frequently occurs in its distal part 

 but may occur in any part or for any extent along the costal arch. 



