You muste ordeyne . . . fastnygis tofore & bihinde & in his flankis. LANFRANC’S CIRURG, 269, c.1400
Development of the posterior body wall
The primitive mesenchyme, as the mesoderm, is differentiated into three layers of the body ( Table 8-1 ). The primitive subcutaneous layer becomes the dermis and the superficial and deep fascias. The body layer forms the skeleton and its support, and the retroperitoneal layer forms three strata involved with the peritoneum and intestines, the urinary organs, and the internal body wall.
| Primitive | Late Fetal | Adult |
|---|---|---|
| Subcutaneous layer | Dermis | Same |
| Superficial fascia | Camper’s fascia | |
| Scarpa’s fascia | ||
| Deep fascia | Same | |
| Body layer | Muscles, ligaments, and bones | Same |
| Retroperitoneal layer | Outer stratum | Transversalis fascia |
| Intermediate stratum (urogenital embedding) | Gerota’s fascia | |
| Inner stratum (intestinal embedding) | Intestinal fascia |
Body wall musculature
Dorsal and ventral division of myotomes
The epaxial, dorsal myotomes form a dorsal division, supplied by a posterior branch of the corresponding spinal nerve ( Fig. 8-1 A). The individual myotomes maintain their segmental positions. They do not migrate but remain in the myotomal region of the developing somite, which is destined to form the extensor muscles of the vertebral column and the lumbar extensor musculature. Moreover, the dorsal myotomes fuse so that by the sixth week, almost all evidence of segmentation is gone. The more distant vertebral levels are connected by the fusion of the fibers from adjacent vertebrae to form longer slips of muscle. An exception is in the deep areas, where the muscles retain their metameric arrangement, connecting one vertebra to the next.
The hypaxial, ventral myotomes form an anterior division, in which each myotome is innervated by the corresponding ventral ramus, as described in Figures 7-2 and 7-3 .
The lateral migratory mass will form the shoulder muscles, including the latissimus dorsi.
The dorsal myotomes lie dorsolateral to the vertebral column and are innervated segmentally by the dorsal ramus of a spinal nerve . The ventral ramus innervates the ventral myotomes ( Fig. 8-1 B). The sympathetic ganglion has connections with the adrenal medulla and the preaortic sympathetic ganglion .
Dorsal muscles of ventral origin
Not all the dorsally situated muscles arise from the posterior myotomes. Before the sixth week, muscles from the anterior myotomes migrate posteriorly to cover the dorsal muscles. The ventral trapezius, deltoid, teres major ( Fig. 8-2 A), and serratus posterior inferior muscles ( Fig. 8-2 B), innervated by anterior rami, cover the dorsal sacrospinalis muscles as they attach to the vertebral spines. Similarly, muscles may migrate in a longitudinal direction, an example of caudal migration being the latissimus dorsi that is innervated by cervical nerves.
Lumbodorsal fascia
The lumbodorsal fascia separates the original deep muscles from those that migrate over them. As it develops with the dorsal muscles, it covers them with sheaths. The sacrospinalis is enclosed between the posterior and middle lamellae of the lumbodorsal fascia, the middle lamella separates the erector spinae from the quadratus lumborum, and the anterior lamella covers the ventral surface of the quadratus lumborum.
Retroperitoneal fascias
The retroperitoneal fascias are not related to the fascias of the dorsal myotomes. The outer stratum is associated with ventral myotomal derivatives. The dorsally and ventrally derived fascias meet at the lateral border of the psoas major, where the muscles developed from the dorsal myotomes (psoas major and quadratus lumborum) and ventral myotomes (transversus abdominis) overlap to form a dense connection.
Retroperitoneal fascial development
The abdominopelvic fascias evolve from one continuous layer of retroperitoneal connective tissue. From this tissue, three fundamental embryonic tissues develop as three strata. One is the mesenchyme, which develops interior to the intrinsic fascia (epimysium) of the body wall musculature. This layer, the outer stratum, forms the abdominal and pelvic fascia, and is represented by the transversalis fascia ( Fig. 8-3 ). A second layer is the loose mesenchyme lying between the outer stratum and the celomic epithelium. This layer is destined to become the intermediate stratum and will form the fascia that encloses the urinary tract. The third layer, the inner stratum, is the connective tissue associated with the celomic epithelium (peritoneum) itself. It will become the fascia involved with the intestinal tract.
Distribution of retroperitoneal fascias
Outer stratum
The outer layer forms the transversalis fascia that lies over the transversus abdominis, quadratus lumborum, and psoas muscles and covers their intrinsic fascia (epimysium) ( Fig. 8-4 ). Fusion takes place between the transversalis fascia associated with the ventral myotomes and the inner lamella of the lumbodorsal fascia, derived with the dorsal myotomes at the lateral border of the psoas muscle. This junction accounts for the difficulty in separating the transversalis fascia from the posterolateral body wall during retroperitoneal mobilization.
Intermediate stratum
Early in development (4½ weeks), the loose mesenchyme in the subserosal layer of retroperitoneal tissue appears, which is the precursor of the extraperitoneal connective tissue that will surround the kidneys and adrenals as well as the aorta, renal vessels, and ureter into the pelvis. At 11 weeks, the mesenchyme differentiates as the intermediate stratum into a laminated covering of the kidney and adrenal that melds with the connective tissue about the aorta and vena cava . As the kidney grows, the fascia becomes more pronounced and splits into two layers, a thicker dorsal layer and a thinner ventral layer. These layers form the posterior lamina and the anterior lamina of the renal fascia (Gerota’s fascia). Between the two layers is the perirenal space that encloses the kidney, ureter, and adrenal. Without development of a kidney, this process does not occur.
Inner stratum
The inner stratum closely invests the gastrointestinal tract beneath the peritoneum. It also constitutes part of the fusion-fascia that forms when an intraperitoneal organ (pancreas, duodenum, or ascending or descending colon) makes contact with the undersurface of the primitive celomic epithelium.
Posterolateral body wall: structure and function
The kidney and adjacent retroperitoneal structures lie close to the posterolateral body wall. An approach from that direction causes less interference with nerves, blood vessels, and abdominal organs than an anterior approach.
Conforming to their embryologic origin, two groups of muscles are found. Muscles derived from the dorsal divisions of the myotomes develop into the back muscles proper, the sacrospinalis group of muscles. Those derived from the ventral divisions form the muscles of the anterior body wall, most of which extend around to the posterior portion of the trunk. These are the external and internal oblique and transversus abdominis muscles. Thus, the muscles of the ventral division are encountered in operations through the flank. The muscles of the anterior body wall have been described in Chapter 7 ; here, the focus is on their posterior extensions exposed through a posterolateral or posterior incision.
Four layers of musculature
The muscles may be divided for purposes of description into four layers: (1) an outer layer , composed of the latissimus dorsi along with the posterior extension of the external oblique and the serratus posterior inferior; (2) a middle layer , made up of the sacrospinalis and the posterior portion of the internal oblique; (3) an inner layer , formed from the quadratus lumborum and iliacus, with a contribution from the transversus abdominis; and (4) an innermost layer , made up of the psoas muscles and the diaphragm.
Outer layer
Structures of the outer muscle layer
The outer layer is composed of the latissimus dorsi, the external oblique, the serratus posterior inferior, the external intercostals, and the posterior lamella of the lumbodorsal fascia. Figure 8-5 A is a posterior view; line X-X indicates the plane of the transverse section shown in Figure 8-5 B.
Latissimus dorsi
Covering most of the lumbar surface of the trunk, this flat, triangular muscle is properly part of the shoulder mechanism; it is attached by a tendon to the humerus and its nerve supply, the thoracodorsal nerve, arises from the brachial plexus. It originates from the spinous processes of the lower six thoracic vertebrae and from the posterior lamella of the lumbodorsal fascia, through which it is ultimately attached to the spines of the lumbar and sacral vertebrae as well as to part of the iliac crest. It is also attached by muscular slips to the lower four ribs that fit between similar slips of the external oblique. The latissimus functions to extend, adduct, and medially rotate the arm as well as retract the shoulder.
External oblique
This muscle arises as seven or eight muscular slips from the outer surface of the eight lower ribs, interdigitating with similar slips of the serratus anterior and the latissimus dorsi. It inserts as a muscle on the anterior half of the iliac crest, contributes one layer to the anterior rectus sheath as an aponeurosis, and inserts on the linea alba, where it fuses with the aponeurosis from the other side. It inserts on the upper border of the symphysis pubis and in the pubic crest. By folding inward, it forms the inguinal ligament (see Figure 9-8 A).
Serratus posterior inferior
This relatively small muscle is partly thoracic and partly lumbar because it arises by an aponeurosis that is fused to the lumbodorsal fascia in an obliquely upward course and from the spinous processes of the two lower thoracic vertebrae and of the two upper lumbar vertebrae. The muscle itself attaches to the lower and outer surfaces of the four lower ribs near their angles, acting to pull the lower ribs outward and downward.
External intercostals
These 11 muscles between 12 ribs cross from the upper border of one rib to the lower border of the next in an oblique direction, running downward and laterally, their insertion being farther forward around the thorax than their origin. They extend from the posterior fibers of the superior costotransverse ligaments at the tubercles to the costal cartilages anteriorly ( Fig. 8-6 A), where they become the external intercostal membrane. They act to approximate the ribs during exhalation and coughing.
