128 THE NERVOUS SYSTEM 



fibres accompany the inferior vena cava into the abdomen, 

 where they are distributed to the liver substance, the gall 

 bladder and the bile ducts. 



As it lies in the root of the neck, the phrenic nerve may 

 become embedded in the deep cervical glands when they are 

 affected with tuberculous disease, or it maybe imolved by 

 the pleuritic thickening which usually accompanies apical 

 phthisis. Under these circumstances, the contractions of the 

 diaphragm may be incomplete and irregular, a condition not 

 uncommon in phthisis in both its early and its later stages. 



The phrenic nerves, like all motor nerves, convey the 

 afferent fibres from the muscles which they supply, and those 

 fibres which the right phrenic supplies to the liver, etc., are also 

 afferent. When the terminal branches of the phrenic are 

 stimulated, the pain may be referred, not to the structure at 

 fault but to the cutaneous distribution of the nerves from 

 which the phrenic takes origin (C. 3, 4 and 5). In tropical 

 abscess of the liver, 16 per cent, of cases are said to experi- 

 ence pain over the right shoulder region (Fig. 61), and 

 the same symptom may be noted, though less commonly, in 

 diaphragmatic pleurisy and cholecystitis (see also p. 190). 



The remaining motor branches supply the prevertebral 

 muscles, including the levator scapula (C. 3 and 4), and 

 assist the accessory nerve to innervate the sterno-mastoid (C. 2 

 and 3) and the trapezius (C. 3 and 4). 



The upper four cervical nerves are rarely involved in 

 injuries, as they are short and not liable to be stretched and 

 torn. Strains of sufficient violence to injure these nerves will 

 probably produce a fracture-dislocation of the cervical vertebral 

 column. 



THE BRACHIAL PLEXUS 



The Brachial Plexus is formed by the anterior rami of 

 the lower four cervical and the first thoracic nerves, and the 

 manner in which these nerves are connected to one another is 

 very constant. 



