386 



eases have been cured by the removal of a portion of the first tho- 

 racic rib, which was therefore apparently in some way causing the 

 symptoms, 



I have lately had the opportunity of examining the excised portion 

 of bone in several such cases, and I was at first surprised to find 

 that it showed no evidence of a well-marked sulcus sub-claviae such 

 as I expected to find in cases where the lowest brachial cord and the 

 first rib had been so closely related. Since then I have published a 

 paper dealing with the influence of vessels and nerves in determining 

 the form of the costal elements at the thoracic inlet and the markings 

 exhibited by these costal elements (2). 



As a result of this investigation I have found that those cases 

 in which the rudimentary rib or the first complete rib has exhibited 

 a well-marked nerve-groove, were not the cases which developed the 

 characteristic pressure symptoms on the lowest nerve trunk. In fact 

 the cases in which the costal elements showed the least-marked nerve 

 grooves were more likely to exhibit symptoms than those in which 

 the grooves were pronounced in character. As the symptoms were 

 clearly of mechanical causation and were cured by the relief of the 

 nerve from pressure on the bone, it appeared to me advisable to 

 study the position of the shoulder in both sexes at various ages. I 

 hoped in this manner to arrive at the true explanation of the me- 

 chanical pressure. 



There are two distinct problems to be considered in the inter- 

 action of bone and nerve. The first was dealt with in a previous 

 paper and embodied the effect of a nerve on pre-osseous tissue (2). 

 The second is considered in the present discussion and deals with 

 the effect of formed bony tissue on nerve. 



The upper limb originally develops as a cervical appendage. 

 During development and extra-uterine life it gradually comes to take 

 up a position posterior to that which it originally occupied. In conse- 

 quence of this movement of the limb the nerve trunks pass caudad- 

 wards to their distribution. 



In its development the arm-bud centred on the seventh cervical 

 segment, hence the central and largest root of the brachial plexus is 

 the seventh cervical nerve. In order to reach its distribution in the 

 limb the ninth nerve passes upward and crosses the thoracic oper- 

 culum, which in normal cases is formed laterally by the first rib. 

 With the caudal movement of the limb the ninth nerve again passes 



