In dogs, lymph drains from tissues throughout the head, including the tonsils, along lymphatic vessels to the facial, parotid, lateral retropharyngeal and mandibular lymph nodes. From the mandibular lymph nodes, lymph may flow to the ipsilateral medial retropharyngeal lymph nodes, or along anastomotic connections to the contralateral node. Afferent lymphatics convey lymph from these nodes to defined areas in the medial retropharyngeal nodes. They divide over the surface of the node, and within trabeculae. Terminal afferent lymphatics are connected to the subcapsular and trabecular sinuses either through circular or oval holes in the vessel wall, or terminate at the sinus where the vessel contains a valve adjacent to the point of entry. The subcapsular sinus surrounds the entire node, and is continuous with an interconnecting network of trabecular and cortical sinuses which convey lymph through the cortex. Connective tissue septa extend through the sinuses and lymph flows freely between adjacent sinuses through holes in the septal walls. Initial efferent lymphatic vessels, which arise from the medullary sinuses between medullary cords, converge towards and unite within the network of medullary trabeculae. Other vessels, which contain valve-like flaps, drain lymph from the subcapsular sinus. Efferent vessels emerge along the hilus and coalesce to form the tracheal trunk. The tracheal trunk has several layers of smooth muscle cells, well developed elastic laminae and connective tissue, surrounding the lymphatic endothelium.