The lumbar aortic lymph nodes (Figures 24: 5, 6; 26: h and 27: 1, 1’, 2, 3, 3’) lie in the region of the lumbar spine, on the aorta and caudal vena cava, and extend from the crura of the diaphragm to the deep circumflex iliac arteries. In the region of the deep circumflex iliac arteries, they are sometimes not clearly demarcated from the medial iliac lymph nodes (see medial iliac lymph nodes). With the exception of the most cranial lumbar aortic lymph nodes (see below), the lumbar aortic lymph nodes may be absent, though notably, in some cases, up to 17 individual lymph nodes were found. The lymph nodes are small, usually very small, with sizes around 1 to 2 mm.
In addition, these lymph nodes are often not clearly defined from the surrounding connective and fatty tissue and are almost always enclosed in fat, resulting in them being quite difficult to find; they may be overlooked if they have not been made visible by injection of their efferent vessels. Therefore, the previous statement that the lumbar aortic lymph nodes were absent in some individual cases should be taken with consideration of this difficulty in identification. For the same reason, I have not reported the absolute and relative weight of the lumbar aortic lymph nodes. The lymph nodes are not arranged in a consistent pattern; instead, they are randomly embedded in a thick pad of fat, and are found dorsally, ventrally, and laterally around the aorta and caudal vena cava, as well as between these vessels. As a result, a more extensive division into subgroups, e.g. into Lnn. retroaortici, praeaortici and paraaortici, or into Lnn. retrovenosi, praevenosi and paravenosi, as in humans, is not possible in dogs.
The lumbar aortic lymph node located most cranially on each side is the most consistent in terms of occurrence, number, and position. As this cranial-most lymph node also has a different afferent drainage area than the other lumbar aortic lymph nodes, it will be termed the cranial lumbar aortic lymph node and discussed in more detail.
The left cranial lumbar aortic lymph node (Figures 27: 1; 28: 3) is occasionally a double lymph node, usually 1 to 2 cm long in large dogs, and is located on both the left crus of the diaphragm and the left lumbar muscles, on the left side of the aorta at the level of, or slightly caudal to, the point at which the cranial mesenteric artery branches off. The lymph node is in contact with the A. and V. lumboabdominalis. This vein is usually found along the ventral side of the lymph node (Figure 28: 1); while the artery is also found along the ventral side of the lymph node, it more often passes closely in front of the lymph node.
The right cranial lumbar aortic lymph node (Figures 27: 1’; 28: 3’) is usually found slightly more caudal, on the right crus of the diaphragm and the right lumbar muscles, and is mostly covered ventrally by the right renal vein (as in Figure 28), though it may also be found more medially under the caudal vena cava (as in Figure 27). One may be inclined to call this lymph node a renal lymph node, but as the afferent drainage area is the same as the left cranial lumbar aortic lymph node (see below), it should be considered a cranial lumbar aortic lymph node, even if it drains lymph vessels from the right kidney. This also applies to the left cranial lumbar aortic lymph node, which also drains some of the lymph vessels of the left kidney. On the left side, there is usually another lymph node that lies on the blood vessels leading to the renal hilum, which one may also be inclined to call a renal lymph node. However, because this lymph node does not have a constant position (and may even be absent), does not drain all the lymph vessels of the left kidney, and cannot be clearly distinguished from the lumbar aortic lymph nodes, I prefer not to consider it as a subgroup (renal lymph node) distinct from the lumbar aortic lymph nodes, and therefore do not discuss it separately as a renal lymph node.
The lumbar aortic lymph nodes drain lymph vessels from the lumbar vertebrae, the lumbar muscles, the M. longissimus dorsi, ileocostalis lumborum, the kidney and ureter, the ovary and uterus, the testes, epididymis and ductus deferens, the tunica vaginalis communis, the M. cremaster, the aorta, the nervous system, and the efferent vessels from the left colic lymph nodes and the medial iliac lymph nodes. Specifically, the cranial lumbar aortic lymph node receives lymph vessels (shown in Figures 27, 28) from the last thoracic vertebrae and last ribs, the lumbar muscles, the M. latissimus dorsi, the Mm. intercostales, the M. longissimus dorsi and iliocostalis, obliquus abdominis externus and internus, transversus abdominis, as well as lymph vessels of the lumbodorsal fascia, mediastinum, costal pleura, and lymph vessels from the diaphragm, peritoneum, liver, kidney, adrenal gland, aorta, and nervous system.
Efferent drainage (Figure 27)
One to 3 efferent vessels leave individual lumbar aortic lymph nodes, draining either into cranially located lumbar aortic lymph nodes, or directly into the cisterna chyli, or joining with the efferent lymph vessels of the other lumbar aortic lymph nodes and the medial iliac lymph nodes to form the pelvic lymphatic trunk (see pelvic lymphatic trunk). The efferent vessels of the cranial lumbar aortic lymph nodes drain directly to the cisterna chyli (Figure 24: 12).