THE LUMBAR PLEXUS. 



313 



descending branch of the first nerve contributes to the genito-crural, often 

 to the anterior crural, and occasionally to the obturator nerve. The second 

 lumbar nerve furnishes the greater part of the genito-crural and external cutaneous 

 nerves, and gives a connecting branch to the third, from which some of the 

 fibres of the anterior crural and obturator nerves are derived. From the third 

 nerve three branches proceed ; the largest part enters the anterior crural nerve ; a 

 small branch, dorsally placed, joins the external cutaneous ; and the third, arising 

 from the ventral aspect of the trunk, passes to the obturator nerve. The fourth 

 nerve also gives three branches, of which the anterior and posterior serve to 

 complete the obturator and anterior crural nerves respectively, while the third, 

 placed below the others, descends to join the fifth lumbar nerve, thus forming the 

 Iwnbo-sacral cord, which enters into the sacral plexus. 



The BRANCHES of the lumbar plexus form two sets, which are distributed, one to 

 the lower part of the wall of the abdomen, the other to the fore part and inner side 

 of the lower limb. In the former set are the ilio-hypogastric and ilio-inguinal 

 nerves, and part of the genito-crural ; and to the latter belong the remaining part 

 of the genito-crural nerve, the external cutaneous, the obturator, and the anterior 

 crural nerves. 



Varieties of the lumbar plexus. In the normal condition, the first three lumbar nerves 

 enter wholly into the lumbar plexus, and the fifth lumbar nerve into the sacral plexus, while 

 the fourth the nervus fur calls of v. Jhering is divided between the two plexuses. The 

 proportion of the fourth nerve, which descends to the sacral plexus, is normally less than one- 

 half, but it varies in different cases from one-twentieth to nine-tenths (Eisler). When the 

 sacral division of this nerve is very large it may be joined by a branch from the third lumbar 

 nerve ; and conversely, when the sacral division of the fourth nerve is very small, the fifth 

 lumbar nerve may send an offset to the lumbar plexus, where it contributes to the formation 

 of the anterior crural and obturator nerves, or, perhaps, only to the former. In these cases 

 there are, therefore, two furcal nerves, viz., 3 I and 4 I, or 4 1 and 5 1 respectively. Farther, the 

 sacral branch of the fourth lumbar nerve may be wanting, and then the fifth lumbar is the 

 sole furcal nerve. A series may thus be traced in which the furcal nerves are progressively, 

 from above downwards, 3 and 4 I, 4 I, 4 and 5 Z, and 5 I. At the same time, the nerves given off to 

 the limb from the lumbar and sacral plexuses are found to vary in their origin to some extent, 

 having a tendency to arise higher or lower, according to the position of the furcal nerve or 

 nerves. In the slighter degrees of variation this is shown by a difference in the size of the 

 contributing roots, so that a given trunk will derive a larger share of its fibres from an upper, 

 and a smaller share from a lower spinal nerve, or vice versa ; but in the more marked degrees 

 the origin may be shifted upwards or downwards to the extent of one spinal nerve. There 

 are thus to be distinguished two extreme forms of plexus (fig. 205), which may be called Ugh 

 and low (the " anterior " and "posterior " forms of Langley, " prefixed " and " postfixed types " 

 of Sherrington), and between which the normal or average plexus is placed, as is shown, for 

 the lumbar plexus only, in the following table, wherein the order of the nerves in each case 

 indicates the size of the roots in descending series, and the roots enclosed in parentheses are 

 inconstant : 



Forms intermediate to these also occur : and the relations of the several offsets of the 

 plexus to one another sometimes vary to a limited extent : that is, one nerve may have a high 

 origin, while the others belong to the normal form, or vice versa. Variations are more 

 frequent and extensive in the downward direction than the opposite. In some instances the 

 extreme forms are associated with irregularities of the vertebral column (see Vol. II, p. 12), 



