Lymphatic trunks and thoracic duct
Lymph nodes and vessels are grouped within the fatty tissue along the margins of the abdominal aorta and inferior vena cava. Lymph draining from them collects into the right and left lumbar trunks (trunci lumbales dexter et sinister), which merge posterior to the major vessels.
Their junction forms the thoracic duct (ductus thoracicus). Most commonly, this occurs at the level of the second lumbar vertebra (less frequently at Th11–Th12). An expansion, the cisterna chyli (cisterna chyli), may form in the lower part of the duct. It is firmly fused with the right crus of the lumbar part of the diaphragm, allowing respiratory contractions of the muscle to assist in pumping lymph. Interestingly, the cisterna chyli is present in only 52% of adults, whereas in individuals with a dolichomorphic body type (42%), a vascular plexus is found in its place. The duct enters the thoracic cavity through the aortic hiatus, running along the posterior wall of the aorta.
Sympathetic trunk and its divisions
The sympathetic trunk (truncus sympathicus) enters the retroperitoneal space through a gap in the lateral part of the diaphragmatic crus. It lies within a duplication of the endoabdominal fascia (fascia endoabdominalis) lateral to the vertebral bodies, near the medial margin of the psoas major muscle (m. psoas major). The right trunk is anteriorly covered by the inferior vena cava.
- Lumbar region: runs along the lateral margin of the aorta. It consists of 2–5 (typically 4) ganglia connected by interganglionic branches. From these, lumbar splanchnic nerves (nn. splanchnici lumbales) branch anteriorly and medially toward the main abdominal plexuses.
- Sacral (pelvic) region: begins inferior to the promontory (promontorium), descending along the anterior surface of the sacrum medial to the anterior sacral foramina. It includes 4 ganglia. It terminates where both trunks unite into a single unpaired coccygeal ganglion — ganglion impar. Branches of this region (sacral splanchnic nerves) form the main autonomic plexus of the pelvis — the inferior hypogastric plexus (plexus hypogastricus inferior / plexus pelvinus).
Autonomic plexuses and parasympathetic innervation
Organs and vessels are innervated by mixed autonomic plexuses, where sympathetic, parasympathetic preganglionic, and visceroafferent fibers converge.
- Abdominal cavity: parasympathetic influence is provided by branches of the vagus nerve (n. vagus), including the posterior vagal trunks.
- Lesser pelvis and descending colon: innervated by pelvic splanchnic nerves (nn. splanchnici pelvini) originating from the sacral spinal cord.
Parasympathetic fibers synapse on postganglionic neurons directly within the ganglia of the organ plexuses. A robust abdominal aortic plexus (plexus aorticus abdominalis) descends along the abdominal aorta, reinforced along its course by branches of the sympathetic trunks and dividing into a series of secondary plexuses.
Nerve topography on the posterior abdominal wall
A complex network of nerves forms within the retroperitoneal adipose tissue and on the muscles of the posterior abdominal wall. They lie in close relation to the muscular apparatus: the quadratus lumborum muscle (m. quadratus lumborum), psoas major and minor muscles (m. psoas major et minor), iliacus muscle (m. iliacus), as well as pelvic muscles (piriformis, obturator internus, levator ani).
Key nerve trunks include:
- Subcostal nerve (n. intercostalis XII).
- Iliohypogastric and ilioinguinal nerves (nn. iliohypogastricus et ilioinguinalis).
- Genitofemoral nerve (n. genitofemoralis), dividing into femoral (r. femoralis) and genital (r. genitalis) branches.
- Lateral cutaneous nerve of the thigh (n. cutaneus femoris lateralis).
- Femoral and obturator nerves (n. femoralis, n. obturatorius).
- Superior gluteal nerve (n. gluteus superior) and sacral plexus (plexus sacralis).
Briefly: answers to topical questions
Into which groups are the lymph nodes of the retroperitoneal space divided?
The lymph nodes of the retroperitoneal space are divided into lumbar, aortic, and caval groups. Located in the adipose tissue along the abdominal aorta are the left lumbar lymph nodes, which include the following groups:
- Lateral aortic — located along the aorta.
- Pre-aortic — belong to the left lumbar nodes.
- Post-aortic — part of the left lumbar nodes.
- Intermediate — part of the left lumbar nodes.
Additionally, lymph nodes are located along the margins of the inferior vena cava and the branches of major vessels.
Which secondary autonomic plexuses arise from the abdominal aortic plexus?
The hypogastric plexuses of the pelvis arise from the abdominal aortic plexus. Descending toward the sacral promontory, its branches form the following secondary autonomic plexuses:
- Superior hypogastric plexus (plexus hypogastricus superior) — diverges laterally and descends into the lesser pelvic cavity.
- Inferior hypogastric plexuses (plexus hypogastrici inferiores dexter et sinister) — formed by branches of the superior hypogastric plexus.
Which nerves contribute to the formation of the inferior hypogastric plexus?
Sympathetic and parasympathetic nerves contribute to the formation of the inferior hypogastric plexus. These include:
- Lumbar splanchnic nerves (nn. splanchnici lumbales) — carry sympathetic fibers.
- Sacral splanchnic nerves (nn. splanchnici sacrales) — sacral splanchnic branches of the sympathetic trunk.
- Pelvic splanchnic nerves (nn. splanchnici pelvini) — contribute the parasympathetic component.
The plexus also includes afferent sensory (visceroafferent) nerve fibers originating from internal organs.
Which nerves are branches of the lumbar plexus?
Branches of the lumbar plexus include nerves involved in the innervation of the anterolateral abdominal wall and scrotum. These are:
- Iliohypogastric nerve (n. iliohypogastricus) — passes between the internal oblique and transverse abdominal muscles, branching in the inguinal and pubic regions.
- Ilioinguinal nerve (n. ilioinguinalis) — emerges from the inguinal canal, becomes superficial, and innervates the skin of the scrotum or labia majora.
To avoid confusing the direction of the sympathetic trunk branches: in the lumbar region they course Anteriorly and Medially (AM), while the sacral region itself lies Medially (M) to the foramina. Remember the "Medial Branches" rule for retroperitoneal sympathetic structures.
Class and exam questions
Where exactly is the thoracic duct formed?
Most commonly, it forms at the level of the second lumbar vertebra by the merger of the right and left lumbar trunks posterior to the aorta and inferior vena cava.
What is the role of the cisterna chyli?
It serves as a reservoir for lymph. Because the cisterna chyli is firmly fused with the right crus of the diaphragm, respiratory muscle contractions literally "squeeze" the lymph upward into the thoracic cavity.
What does the sympathetic trunk terminate as?
The right and left sympathetic trunks descend into the pelvis and unite at the coccygeal level into a single unpaired ganglion — the ganglion impar.
Where do the pelvic organs receive their parasympathetic innervation from?
The source is the pelvic splanchnic nerves (nn. splanchnici pelvini), which arise from the sacral part of the spinal cord.
What else to study on this topic
- Topography of the branches of the genitofemoral nerve (r. femoralis and r. genitalis).
- Secondary autonomic plexuses branching from the abdominal aortic plexus.
- Formation and branches of the sacral plexus (plexus sacralis).
- Course and syntopy of the posterior vagal trunks in the retroperitoneal space.
- Relationship between the right sympathetic trunk and the inferior vena cava.
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