The lymph vessels of the upper lip (see Figure 13) all drain to both groups of the mandibular lymph nodes. Most of the lymph vessels lie between the skin and the skin muscle, though a smaller number may also penetrate the skin muscle to the inner surface of the muscle. Some lymph vessels run in large, caudodorsally convex arcs along the M. masseter, while others run in the intermandibular region (the region between the left and right sides of the body of the mandible).
Interestingly, some of the lymph vessels in the intermandibular region cross the median plane, resulting in the lymph vessels from the left half of the upper lip draining to the right mandibular lymph nodes, and vice versa. The lymph vessels of the lower lip also drain to the mandibular lymph nodes; these lymph vessels join those from the skin of the intermandibular region (see Figure 13), and some cross the median plane, so that they drain to the lymph nodes on the contralateral side. These behaviours apply to the lymph vessels of all the structures of the lips (skin, musculature, and mucosa). The lymph vessels of the mucosa of the upper lip tend to emerge between the end of the M. zygomaticus and the M. levator nasolabialis, while those of the mucosa of the lower lip emerge at the ventral edge of the cheek muscle, near the corner of the lip, and then proceed to join the other lymph vessels.
The lymph vessels of both lips were injected 11 times, and in no case did any of the lymph vessels drain to the parotid lymph node, not even from the upper lip. I mention this negative finding because, in the cow, the lymph vessels of both lips drain to the mandibular lymph nodes as well as the parotid lymph node, and even occasionally to the lateral retropharyngeal lymph node. As already mentioned, the lymph vessels of the lips frequently cross the median plane, and I was able to confirm this in smaller dogs with injections made 1 cm lateral to the median plane in one of the lips.