752 THE NEBVOUS SYSTEM. 



B. Cutaneous Distribution. Lower Limb. 



VARIATIONS IN THE POSITION OF THE LIMB-PLEXUSES. 



Two different kinds of variations occur in relation to the limb -nerves. 



(1) Individual variations, both in the extent of origin and in the area of distribution of a 

 given nerve, are not uncommon ; these variations are usually concomitant with compensatory 

 variations in adjacent nerves, and are due to the fibres of a given spinal nerve taking an 

 abnormal course in the trunk of another nerve of distribution and effecting a communication 

 with the proper nerve peripherally. In this way the variations in the origin and distribution 

 of the intercosto-brachial nerve may be explained ; and, similarly, the ulnar nerve may have 

 some of its fibres carried as far as the forearm, incorporated with the median and transferred to 

 it by a communication between the two nerves in that region. 



(2) Variations in the limb-plexus, in relation to the vertebral column, are the chief cause of 

 variations in the constitution of the limb-nerves. These variations affect more or less the whole 

 series of nerves in the plexus. 



The brachial plexus is subject only to very slight variation in position and arrangement. 

 It may be reinforced at the upper end by a slender trunk from the fourth cervical nerve, and, 

 more frequently, by an intra - thoracic communication between the second and first thoracic 

 nerves. The presence of one or other of these nerves is an indication of a slight tendency 

 towards a cephalic or caudal shifting of the whole plexus in relation to the spinal medulla. It is, 

 however, never sunicient to cause the exclusion to any extent of the nerves normally implicated. 

 The presence of a cervical rib may coincide with little or no change in the relation of the nerves. 

 Indeed, the inclusion of the second thoracic nerve in the plexus may be, as already stated, 

 merely an individual variation, a change in the path to the limb of the intercosto-brachial 

 nerve. Concomitant variations occur among groups of nerves, however, which indicate a certain 

 tendency to variation in the position of the whole plexus. At one end, the suprascapular and 

 niusculo-cutaneous nerves may arise from the fourth and fifth, fifth alone, or fifth and sixth 

 cervical nerves. At the other end of the plexus, the radial may or may not receive a root 

 from the first thoracic nerve, and this addition is rather more likely to occur when the second 

 thoracic nerve is implicated in the plexus. 



The lumbo-sacral plexus shows a very considerable variability in position and constitution. 

 Eisler records concomitant variations in the plexus in 18 per cent, of the cases examined by him. 

 The variations occur within wide limits. The plexus may begin at the eleventh or twelfth 



