LIMB-NERVES AND PLEXUSES. 383 



not the whole, of both the lumbar and sacral plexuses, and the name e rural plc^u* may be 

 used for the sum of the nerves constituting the proper limb-plexus. Into this enter usually 

 seven nerves, viz., the lower four lumbar and the upper three sacral, but the first lumbar 

 nerve in some cases also furnishes a branch, in rare instances even the last dorsal nerve ; 

 while at the opposite end the offset from the third sacral may be wanting, or in extreme cases 

 of low form of plexus the fourth sacral may be drawn in (cf. pp. 313 and 325). 



In the case of the brachial plexus all the nerves have the same relation to the shoulder- 

 girdle, passing behind the clavicle. With the crural plexus the arrangement is different : in 

 association with the greater development of the ventral portion of the limb-girdle the nerves 

 are separated as they enter the limb, one set, the external cutaneous and anterior crural, 

 passing in front of the pubic portion of the girdle, a second, the obturator, passing between 

 the pubic and ischial portions of the girdle, and a third, the largest, the gluteal, sciatic, &c., 

 passing behind the ischial portion of the girdle- the prezonal, diazonal, and m eta-zonal ncrres 

 of Fiirbringer. Thus there is brought about the separation of the lumbar and sacral plexuses 

 of descriptive anatomy ; and the nercusfurcalis is only the nerve (entering into loops like the 

 others) which lies at the boundary between the pre- and diazonal nerves on the one hand and 

 the metazonal nerves on the other. 



As to the significance of the plexuses, it may be remarked in the first place that the term 

 " plexus " is somewhat misleading. Except in the case of the brachial plexus, the so-called 

 " plexuses " are not interlacements of the spinal nerves, but result from the multiple origin 

 of the nerves of distribution, so that most of the offsets of the plexus contain fibres derived 

 from two or more segmental nerves. This multiple origin is intimately related to the 

 fusion of the myomeres from which the muscles of the limbs are derived, and in association 

 with this is the multiple innervation of individual muscles (see p. 354). A similar condition 

 is seen in the lower intercostal nerves (p. 310) before supplying the broad muscles of the 

 abdomen, in which the constituent myomeric elements are fused, so that these muscles are 

 polymeric and pott/neural; whereas such communications are much less frequent between the 

 upper intercostal nerves, which are distributed to the mono tit eric and moiwneural intercostal 

 muscles. &c. Similarly also with the cutaneous offsets, the plexuses afford the means by which 

 the fibres of two or more segmental nerves become mingled in order that they may supply in 

 common a given area of skin. Gegenbaur and Fiirbringer look upon the plexuses of the limbs 

 as the result of the shifting of the latter along the vertebral column in the course of phylo- 

 genetic development, but it is extremely doubtful whether this shifting of the limbs has 

 taken place to the extent supposed by them, and it is difficult to explain in this way the 

 presence of the communications between the lower intercostal nerves while they are absent 

 between the upper nerves, or the formation of the pudendal plexus. It does not appear that 

 the plexuses have a physiological significance, so far at least as the muscles are concerned, 

 beyond the provision of the multiple supply, that is, the mingling of the nerves in the plexuses 

 does not seem to be necessarily related to any functional grouping of the muscles supplied. 



The nerves entering the limb-plexuses show a remarkable division, the significance of 

 which was first pointed out by Paterson, into anterior or ventral and posterior or dorsal port ions, 

 corresponding to the primary subdivisions of the limb-musculature (Vol. II, pp. 274, 275). 

 The dorsal and ventral divisions of a nerve entering the limb are regarded by Paterson and 

 others, who consider that the limb-nerve represents the whole anterior primary division of a 

 spinal nerve, as corresponding respectively to the lateral offset and the anterior portion of a 

 segmental nerve. The ventral divisions of the brachial nerves form the inner and outer cords 

 of the plexus, and the dorsal divisions the posterior cord of the plexus. The ventral divisions 

 of the crural nerves include the obturator, internal popliteal, and some smaller branches, 

 while the dorsal divisions furnish the external cutaneous, anterior crural, external popliteal, 

 gluteal, and some other nerves (p. 325). In their cutaneous distribution, while as a general 

 rule the skin overlying ventral muscles is supplied by nerves of ventral origin, and that over 

 dorsal muscles by the dorsal divisions of the nerves, the limits are not strictly kept, and the 

 territory of the one group is often greatly extended at the expense of the other (cf. figs. 221, 

 222, and 223). In the following table the nerves of each group are shown, together with the 

 probable homologies of the nerves of the two limbs, so far as they can be traced : 



CLASSIFIED TABLE OF THE NERVES OF THE LIMBS, WITH THEIR 



PROBABLE HOMOLOGIES. 



UPPER LIMB. LOWER LIMB. 



Dorsal nerres. Dorsal nerves. 



N. to rhomboidei. 

 Posterior thoracic. 



Suprascapular Branches of anterior crural to iliacus. 



N ( Superior gluteal. 



Subscapular (3) P pyrff ormis. 



Circumflex : muscular part ... J { Inferior gluteal. 



B B 2 



