The colic lymph nodes (Figure 26: e, f, f, g, g) are the large intestinal lymph nodes. Since no lymph nodes are found in the ileocecal mesentery, the mesocecum, or the mesorectum, the lymph nodes draining the large intestine are limited to the lymph nodes of the colon, or the colic lymph nodes. The colic lymph nodes can be found individually, or in groups, between the sheets of the short mesocolon or mesorectum. There are anywhere from 3 to 8 such lymph nodes, ranging from 3 mm to 2.5 cm in length. The colic lymph nodes are divided into the right colic, the middle colic, and the left colic lymph node groups.
A. Right Colic Lymph Nodes
The right colic lymph node (Figures 25: 5, 26: e) is an occasionally doubled lymph node which lies just behind the origin of the ascending colon (Figure 26: 7), between the sheets of the mesocolon where the ileocolic vein branches, and more specifically, to the left of the border between the cecum (Figure 26: 6), the ascending colon (Figure 26: 1), and the ileum (Figure 26: 8), at the origin of the branches of the ileocolic vein.
In 25 examined cases, there was 1 lymph node in 19 cases, 2 lymph nodes in 5 cases, and 3 lymph nodes in 1 case.
The efferent vessels of the right colic lymph node (Figure 26: e) join with the efferent vessels of the jejunal lymph nodes to form an extensive vascular network, from which the lymph vessels converge to form the intestinal trunk (see intestinal trunk).
B. Middle Colic Lymph Nodes
The middle colic lymph node (Figures 25: 7; 26: f, f) is found in the mesentery of the transverse colon (Figure 26: 8), either where the left colic vein joins the middle colic vein (Figure 26: 12), or slightly towards the portal vein from this location. If there is only 1 lymph node, then it is usually situated either against the caudal border of the vein or on the vein itself, and if there are 2 lymph nodes, they are then situated on both borders of the vein (as shown in Figure 26). Usually, the lymph node is located relatively far from the transverse colon, in the mesentery near the jejunal lymph nodes (5 to 7 cm from the transverse colon in large dogs, when the mesentery is tightened), and is sometimes covered on the ventral side by the jejunal lymph nodes (Figure 25: 62). If there are 2 or 3 middle colic lymph nodes and also a right colic lymph node, then the 2 lymph node groups may almost border each other. In 25 examined cases, the middle colic lymph nodes were absent in 6 cases, 1 lymph node was present in 12 cases, 2 lymph nodes were present in 5 cases, and 3 or 4 lymph nodes were present in 1 case each.
Efferent drainage (Figure 26: f, f)
If there are several lymph nodes, then the individual lymph nodes of the group are usually connected to one another through their efferent vessels. Additionally, several (2 to 4) efferent vessels arise from each lymph node and merge with their neighbouring efferent lymph vessels. The vessels formed in this way drain to the intestinal trunk (Figure 26), and thus these lymph nodes directly drain into the cisterna chyli.
C. Left Colic Lymph Nodes
The left colic lymph nodes (Figure 26: g, g) are a group of 2 to 5 lymph nodes, which are in the mesocolon near the mesorectum, where the caudal mesenteric artery and its terminal branches approach the intestine. They are sometimes either found slightly dorsal to the cranial hemorrhoidal artery, which runs towards the thorax, sometimes on the left colic vein near the intestine (in large dogs, when the mesentery is tightened, the lymph node is 2 to 3 cm away from the intestine) or sometimes in the angle between these two vessels.
efferent drainage (Figure 26: g, g)
The individual lymph nodes of the group are usually connected to one another, and only once did I fail to identify these connections through injection of dye solutions. In addition, 1 to 3 efferent vessels emerge from each lymph node, and flow into the medial iliac lymph node (Figure 26: i), the lumbar aortic lymph nodes (Figure 26: h), the middle colic lymph node (Figure 26: f), and occasionally directly into the intestinal trunk (Figure 26: n). The 1 to 2 lymph vessels leading to the middle colic lymph node run irregularly in the mesentery or merge with the corresponding lymph vessels draining the descending colon.
Afferent drainage of all colic lymph nodes
The colic lymph nodes all drain the lymph vessels of the ileum, cecum, and colon, and the efferent vessels of the left colic lymph nodes drain to the middle colic lymph node.
The absolute weight of the lymph nodes described under Section 1.7 (including the hepatic, splenic, gastric, omental, duodenal, jejunal, and colic lymph nodes), ranged between 0.78 and 27.94 g, and the relative weight between 0.0159% and 1.433%