260 



(3) The rib may be entirely cut off and separated from tlie sternum, 

 so that no ligamentous band connects the two. This condition 

 was illustrated by case B. Fig. III. 



My observations indicate that this last modification is more usually 

 associated with the influence of the nerve and results in the presence 

 of a small rudimentary rib. 



There is no need in this paper to enter into discussion concerning 

 the primary development of the nerves and vessels, and their im- 

 portance in the formation of the skeleton. This subject has been dealt 

 with by Wood Jones (3) and has also been referred to in a previous 

 paper of my own. (1). 



For purposes of classification the variations of the thoracic inlet 

 may be sub-divided into two divisions, those of ascending and des- 

 cending scales. 



The ascending scale involves the presence of a rib more or less 

 rudimentary in form and associated with the seventh vertebra. 



Under the heading of the descending scale are those conditions 

 which exhibit deformation of the normal first rib and which may 

 result in its being transformed more and more into a rudimentary 

 element. 



At this juncture it may be well to state that the letters N, 

 A, V, in the illustrations reproduced in this article, indicate the site 

 of the impression produced on the bone by the crossing of the nerve 

 trunk and vessels. 



Thus N refers to the lowest trunk of the brachial plexus: A to 

 the sub-clavian artery: and Y to the sub-clavian vein. 



All the illustrations of this article were drawn natural size. 



It was intended that they should be reduced to scale in order 

 to render them comparable with each other. 



Owing to a misunderstanding Figs. I and II have been reproduced 

 ^/i natural size, while the other illustrations are ^/o natural size. 



I. Types in the ascending scale. 

 Taking first the variations in the ascending scale, the absence of 

 grooves on the first rib is found to be associated with certain modi- 

 fication of the seventh cervical vertebra and the presence in more 

 or less complete form of an additional rib element — the seventh 

 cervical rib. 



