THE BRACHIAL PLEXUS 159 



the plexus, however, may involve the trunks or cords, and 

 certain facts must be borne in mind when the site of any 

 such lesion is being determined. 



When the fifth cervical segment is involved, the actions of the 

 rhomboids and the supra- and infra-spinati must be carefully 

 investigated. If the rhomboids are found to be paralysed, 

 then the nerve must be affected close to the intervertebral 

 foramen. If the rhomboids are not involved and the supra- 

 and infra-spinati are paralysed, then the lesion must have caught 

 the nerve just prior to the formation of the upper trunk of the 

 plexus, but if both groups have escaped, the lesion is one of 

 the upper trunk. Unfortunately, it is not easy to ascertain 

 whether these groups are paralysed or have escaped. In the 

 case of the rhomboids, the position of the scapula is altered 

 somewhat on the affected side. It occupies a slightly lower 

 position and, the bone being rotated clockwise (as viewed from 

 in front) by the serratus anterior, the inferior angle is farther from 

 the median plane than it is on the sound side. The examination 

 of the supra-spinatus is rendered difficult because the muscle is 

 almost entirely covered by the trapezius and is nowhere sub- 

 cutaneous. Fortunately, sufficient of the infra-spinatus is 

 exposed to enable its electrical reactions to be examined. 



When the sixth segment is involved, the action of the serratus 

 anterior must be tested. Forward pushing movements against 

 resistance make the digitations of origin stand out prominently 

 on the lateral thoracic wall, in moderately well-developed 

 subjects. Ability to flex the shoulder beyond an angle of 

 90 indicates that the serratus anterior is acting normally. If 

 the serratus anterior is paralysed (p. 132), the sixth cervical nerve 

 is involved close to its exit from the intervertebral foramen. 



When the first thoracic segment is involved, the condition 

 of the cervical sympathetic gives a clue to the situation of 

 the lesion. When it is affected, the lesion must be situated 

 between the point where the first thoracic nerve gives off its 

 white ramus communicans and the point at which it leaves 

 the vertebral canal. 



