[The courses and the segmental origins of the cutaneous branches of the thoracic dorsal rami]

Abstract

It is described in many textbooks that the medial cutaneous branches (RCM) from the medial branches of the upper six thoracic dorsal rami supply the upper half of the back of the body, the lateral cutaneous branches (RCL) from the lateral branches of the lower six thoracic dorsal rami supply the lower half of it, and the area supplied by both branches is limited to a few segments. Unlike those descriptions, we had frequently observed RCL from the second or the third thoracic dorsal ramus penetrating the rhomboideus muscle during previous research concerning the double innervation of the superficial muscles of the back by both the ventral and the dorsal rami (Kumaki et al., 1984). To make clear the origin of the discrepancy between the description in the textbooks and our observations, we examined the segmental origins, the courses, and the distributions of both the medial and the lateral branches of the thoracic dorsal rami on 20 sides of 11 bodies dissected in the years 1986 and 1989. Consequently, RCL from the second thoracic dorsal ramus (Th 2) was observed in 25% of the cases and RCL from Th 3 and Th 4 were observed in 50% and in 70%, respectively. The highest segment of RCL was Th 2 in 25%, Th 3 in 25%, Th 4 in 35%, Th 6 in 10%, or Th 8 in 5%, and the mean was Th 3.65 +/- 1.53. On the other hand, the lowest segment of RCM was Th 6 (15%), Th 7 (35%), Th 8 (25%), Th 9 (15%), or Th 10 (10%), and the mean was Th 7.70 +/- 1.19. The mean number of the segments at which the dorsal ramus of the thoracic nerve sent both RCM and RCL was 4.55 +/- 1.50 (Max: 9 segments). Thus, we made clear that RCL from the upper thoracic nerves were commonly observed and that the number of segments sending both RCM and RCL was larger than hitherto described. The course of the upper RCL was bent at the points where the RCL penetrated the superficial muscles of the back forming a "Z"-shape, i.e., RCL changed its course from an infero-lateral to an infero-medial direction at the point of penetrating the rhomboid muscle and from an infero-medial to a lateral direction at the point of penetrating the trapezius muscle or the latissimus dorsi muscle. These directions might be associated with the development of the muscles. Sometimes RCL was sharply pulled in a medial direction by the trapezius muscle to penetrate the muscle near the median plane and appeared as RCM. Therefore, we supposed that the main reason why the upper RCL had been overlooked was because the complicated zigzag course of the RCL was damaged by the inadequate dissection or RCL was mistaken for the RCM. While the points where the cutaneous branches penetrated the superficial muscles were variable, the points where they penetrated the thoraco-lumbar fascia were relatively stable. This point of the RCM was generally near the tip of the spinal process of the same segmental number of the nerve, and the same point of RCL was generally at the gap between the longissimus and the iliocostal muscles in the intercostal space one segment lower than the segment of the RCL. The RCL from the last thoracic to the third lumbar dorsal rami communicated with one another to form a nerve plexus under the lumbo-dorsal aponeurosis, then penetrated that aponeurosis forming several nerve bundles, crossed over the iliac crest and supplied the hip skin as the superior cluneal nerves. Therefore, each bundle was not equivalent to each segment, but was composed of two or more segments.