259 



In other five, however, the characters of the groove suggested 

 that nerve influence had been paramount. In these latter cases the 

 nerve groove was found more marked on the right side. 



These observations were made quite independently of these in 

 a former paper (1), but confirm the conclusions to which I came last 

 year. The ribs in addition presented a moderate hollowing on their 

 upper surface corresponding in position with that of the sub-clavian 

 vein. This was not found to be more prominent on the left side so 

 constantly as was the arterial groove. That this vein marking is a 

 hollowing more than a distinct groove is due to the moniliform con- 

 dition of the vessel in this situation. The valve which is present on 

 the distal side of the opening of the external jugular vein (2) converts 

 the sub-clavian vessel into a reservoir rather than a tube and the 

 rib bears a corresponding marking. 



To sum up the preceding statements, the majority of specimens 

 of first dorsal rib present: 



(1) A sulcus sub-claviae which is frequently indeterminate in 

 character and which makes but slight impression on the bone. 



(2) An impression for the sub-sclavian vein which is also com- 

 paratively poorly marked and has the character of a hollowing of the 

 upper surface rather than of a distinct groove. 



The most frequent and at the same time slighter variations from 

 the, above condition are in the degree of intensity of the markings. 

 These may disappear altogether or may become deeper. In the former 

 case the seventh cervical vertebra may be observed to show a rudiment 

 of a rib element. In the latter instance the grooves may become so 

 deep as to modify very considerably the shape and appearance of 

 the bone. 



The influence of a vessel or nerve on the rib in this situation 

 may be expressed in one of three modifications: 



(1) It may cause compression of the segmental skeleton so that the 

 highest rib becomes fused with the second of the series and is 

 thus connected with the sternum only through the substance of 

 its neighbour (see Fig. lY). 



(2) It may cause the rib to be only partly composed of osseous 

 tissue, the remaining portion being formed of ligament. In this 

 case the rudimentary rib is connected with the sternum by a 

 ligamentous band, as was the condition presented by case A. 

 Fig. V. 



17* 



