The deep inguinal lymph node (Figure 27: 8) is a small lymph node that is found inconsistently in a minority (about one-third) of cases. Of 50 examined dogs, it was only found in 18. Of these 18 dogs, it was found on both sides (5 dogs), only on the left (1 dog), or only on the right (12 dogs), while in the remaining 32 dogs, it was absent. The lymph node is 2 to 11 mm in size and lies on the ventral surface of the tendon of the M. psoas minor, at the point where the muscle attaches to the ileopectineal crest, just caudal to the external iliac vein (Figure 27: k). In regard to the surrounding vessels, the lymph node lies at the angle formed by the external iliac vein and the hypogastric vein (Figure 27: l’) before both vessels join the common iliac vein (without extending to the apex of the angle), and just dorsal to the branch of the deep femoral artery.
The identification of this lymph node is uncertain. Merzdorf  has identified it as a lateral hypogastric lymph node but subsequently added: “whether this is a deep inguinal lymph node is an unanswered question”. Ellenberger-Baum  and Chauveau-Arloing  both include this lymph node under the classification of medial iliac lymph nodes. I instead identify the lymph node as a deep inguinal lymph node because the afferent drainage area corresponds to that of the deep inguinal lymph node identified in cattle (see Baum , page 41). The deep inguinal lymph node should be considered an accessory to the medial iliac lymph node group, or at least as a lymph node split off from the medial iliac group, because when the deep inguinal lymph node is absent its afferent vessels drain directly to the medial iliac lymph nodes in both the dog and the cow. Despite this, it should be given a separate name, because it corresponds to the deep inguinal lymph nodes present in some animal species (e.g. horses) and is located in the femoral canal.
The deep inguinal lymph node drains some of the lymph vessels ascending in the femoral canal. In the absence of the deep inguinal lymph node, these vessels drain directly to the medial iliac lymph nodes. This behaviour was demonstrated for the lymph vessels draining the following structures: the Mm. glutei, the M. tensor fasciae latae, biceps, semitendinosus, semimembranosus, gracilis, pectineus, adductor, sartorius, quadratus femoris, obturatorius externus, quadriceps, M. tibialis anterior and tendon, M. extensor digitalis longus and lateralis, Mm. peronaei, M. gastrocnemius and tendon, M. flexor digitalis sublimis and profundus and tendons, M. popliteus, Mm. interossei pedis, M. extensor digitalis pedis brevis and M. obliquus abdominis internus, the lymph vessels of the fascia lata and cruris, of the hip, stifle and tarsal joints and of the femur, patella, tibia, fibula, tarsal and metatarsal bones and the peritoneum, and the efferent lymph vessels from the popliteal lymph node and the superficial inguinal lymph nodes. Often, however, no lymph vessels from the above structures drained into the deep inguinal lymph node, even when the lymph node was present.
Several efferent vessels emerge from the deep inguinal lymph node. They immediately merge to form 1 to 2 vessels, which travel over the external iliac vein and the external iliac artery (both surfaces) to the medial iliac lymph node, and occasionally also to the hypogastric lymph nodes, as shown in Figure 27: 8 and Figure 37: f’.