126 THE NERVOUS SYSTEM 



may also involve the posterior rami. In cases of gastric ulcer 

 or ureteral calculus, areas of cutaneous hyperalgesia are fre- 

 quently to be found in the regions supplied by the posterior 

 rami. Sometimes, however, the pain is not referred to the 

 skin, but to the sensory nerve-endings in the muscles of the 

 back, and it then gives rise to areas of muscular hyperalgesia 

 which can readily be recognised, if the finger is carried down- 

 wards over the sacro-spinalis (erector spinse). Gentle pressure 

 is sufficient to make the patient wince in quite a characteristic 

 way when the finger passes over the hyperalgesic area. 



THE ANTERIOR RAMI (PRIMARY DIVISIONS) 



The anterior rami of the upper four cervical nerves take 

 part in the formation of the Cervical Plexus, which is placed 

 under cover of the sterno-mastoid muscle. 



The cutaneous branches of this plexus supply a large area of 

 skin, extending downwards on the trunk to the level of the 

 second rib (Fig. 61), where the branches of the third and 

 fourth cervical nerves overlap the branches of the second 

 thoracic. The intervening nerves (C. 5-8, T. i) do not 

 appear on the surface of the trunk, as they are destined for 

 the supply of the upper limb. On this account, the line of 

 anaesthesia on the anterior surface of the body is the same 

 for all fracture-dislocations of the vertebral column occurring 

 between the fourth cervical and the first thoracic vertebrae, and 

 it therefore bears no relation to the site of the injury. The 

 cervical plexus (C. 3 and 4) is responsible for the supply of the 

 skin over the acromion and over the proximal part of the 

 deltoid (vide infra). 



Of the motor branches of the cervical plexus, the Phrenic 

 Nerve is the most important. Most of its fibres come from 

 the fourth cervical nerve, but it usually receives a few fibres 

 either from the third or the fifth, in addition. The phrenic 

 nerve descends through the neck, lying behind the internal 

 jugular vein, and comes into intimate relation with the lower 



