POSTERIOR PRIMARY DIVISIONS. 281 



lower dorsal nerves are placed between the multifidus spinae and longissimus dorsi, 

 and end in the multifidus without giving branches to the integument. Where 

 cutaneous nerves are supplied by the internal branches, there are generally none 

 from the external branches of the same nerves, and vice versa. 



The cutaneous offsets of the dorsal nerves run obliquely downwards in their passage 

 between and through the muscles, so that they become superficial and are distributed to the 

 skin at a level below that of the vertebra to which they correspond (see fig-. 189). This 

 descent is but slight in the case of the upper branches, which are derived from the internal 

 divisions of the nerves, and which in their subcutaneous course are directed nearly horizontally 

 outwards, but it becomes progressively greater in the lower branches, proceeding- from the 

 outer divisions, and the offset of the twelfth dorsal only makes its appearance a little distance 

 above the iliac crest. There is considerable variety in the size and in the extent of distribution 

 of the several nerves, which often differ on the two sides of the body ; and twigs from one or 

 two of the lowest dorsal nerves frequently pass over the iliac crest to the skin of the buttock. 

 (See Wardrop Griffith and Oliver, "On the Distribution of the Cutaneous Nerves of the Trunk," 

 Proc. Anatom. Soc., 1890, in Journ. Anat., xxiv.) 



Varieties. The cutaneous offset of the first dorsal nerve is sometimes absent. There are 

 not unfrequently cutaneous branches from both divisions of the middle (sixth, seventh, and 

 eighth) dorsal nerves. 



LUMBAR NERVES. The external branches enter the erector spinae, and give 

 branches to that muscle. From the upper three, cutaneous nerves are supplied ; 

 and from the last, a filament descends to the corresponding branch of the first 

 sacral nerve. The cutaneous nerves given from the external branches of the first 

 three lumbar nerves (nn. clunium superiores) pierce the fleshy part of the ilio- 

 costalis and the aponeurosis of the latissimus dorsi : they cross the iliac crest near 

 the edge of the erector spinse, and terminate in the integument of the gluteal region. 

 One or more of the filaments may be traced as far as the great trochanter of the 

 femur. 



The internal branches wind backwards in grooves close below the mamillary 

 processes of the vertebrae, and sink into the multifidus spinse muscle. 



SACRAL NERVES. The posterior divisions of these nerves, except the last, issue 

 from the sacrum, through its posterior foramina. The first three are' covered at 

 their exit from the bone by the multifidus spinae muscle, and bifurcate like the 

 posterior trunks of the other spinal nerves ; but the remaining two, which continue 

 below that muscle, are not thus divided. 



The internal branches of the first three sacral nerves are small, and are lost in 

 the multifidus spinae muscle. 



The external branches of the same nerves are united with one another, and 

 with the last lumbar and fourth sacral nerves, so as to form a series of anastomotic 

 loops on the upper part of the sacrum. From these, branches are then directed 

 outwards to the cutaneous or posterior surface of the great sacro-sciatic ligament, 

 where, covered by the gluteus maximus muscle, they form a second series of loops, 

 and end as cutaneous nerves (nn. clunium medii). The latter pierce the great 

 gluteus muscle in a line drawn from the posterior superior iliac spine to the tip of 

 the coccyx. They are commonly two in number one is near the lower part of the 

 sacrum, the other by the side of the coccyx. All are directed outwards over the 

 great gluteal muscle. 



In six dissections by Ellis the above arrangement was found to be the most frequent. The 

 variations to which it is liable are these : the first nerve may not take part in the formation 

 of the second series of loops, and the fourth may be associated with them. From the first 

 three sacral nerves filaments are given to the sacro-iliac articulation (Luschka, Rudinger). 

 Small cutaneous twigs from the internal branches of the lower lumbar and upper sacral nerves 

 are described by many anatomists (fig. 188, .s-V). 



The posterior divisions of the last two sacral nerves are smaller than those above 

 them, and are not divided into external and internal branches. They are connected 



