This study of the canine lymphatic system was carried out using the same methods as those applied to cattle (see Baum ), although the research was performed independently. As in cattle, the lymph vessels of all organs in dogs were macroscopically traced when injected, with the exception of small portions of the bones and joints, the middle and inner ear, the muscles of the eye, and the eyeball (bulbus oculi). All the lymph nodes, their drainage areas, and their efferent duct behaviour were carefully examined, using the techniques described in The Lymphatic System of Cattle , pages 3 to 10 (see the Methodology section for an excerpt from this publication).
The available literature has been considered but is of limited use due to its mostly general nature. This applies to the veterinary anatomical works Anatomy of the Dog by Ellenberger-Baum , Anatomy of Domestic Animals by Ellenberger-Baum , Anatomy of Domestic Animals by Martin , Anatomy by Chauveau-Arloing , and Anatomic Images of Domestic Mammals, 1829, by Gurlt . These texts provide detailed canine anatomy but are not always specifically cited in this manuscript as the lymphatic system information is too general and furthermore outdated by my extensive investigations. There are several more specific publications on lymph nodes and lymph vessels in dogs listed in the bibliography at the end of this work; including multiple articles that I have published [3 – 13]. The content of my previous work is of course considered in every respect in the current publication. Of them, Merzdorf’s research on the lymph nodes of the dog is prominent . It was carried out as a preparation for the current comprehensive publication at my institute, under my direction, and therefore the material from those studies included in this work are not specifically referenced.
The number of lymph nodes in dogs is relatively small, both in terms of the number of lymph node groups and the number of lymph nodes in each individual group. This results in dogs being the domestic animal with the smallest number of lymph nodes and lymph node groups. The sizes of the lymph nodes vary between 1 mm to 7.5 cm (though, in notable cases, the jejunal lymph nodes of large dogs may be up to 20 cm in size). However, the number of lymph nodes over 2 cm in length is very low, even in large dogs. The lymph nodes of the dog are relatively large when compared to the lymph nodes of humans, horses, and pigs.
The shape of lymph nodes in the dog fluctuates widely. The majority of lymph nodes are round or oval, or elongated or bean-shaped, and somewhat flattened. There are additionally a number of irregular shapes: some lymph nodes are dumbbell-shaped, lobed, S-shaped, or horseshoe-shaped (see middle tracheobronchial lymph nodes), or very elongated and extremely flattened, almost ribbon-shaped (see hepatic lymph nodes).
A hilus appearing macroscopically as an obvious indentation is not present in all lymph nodes, at most present in about 50 to 60% of canine lymph nodes. In about another 20% of lymph nodes, the hilus appears only as a slight inward projection of the capsule. In elongated lymph nodes, it usually appears as a longitudinal groove on one side of the lymph node.
According to my observations, the number of efferent vessels of the dog’s lymph nodes varies between 1 and 10, but is most commonly between 1 and 3. This does not always mean that only 1 to 3 efferent vessels emerge from a particular lymph node – there can be many more tiny efferent lymph vessels emerging from a lymph node, at times upwards of 50. However, these tiny vessels unite to form 2, 3, or up to 10 larger vessels, usually so quickly that one can hardly distinguish and count the tiny vessels when they first emerge from the lymph node. Larger numbers of vessels (6 to 10) will again combine to form 3 to 5 larger vessels. In most cases, the exact number of efferent vessels cannot be counted because the vessels continuously divide and reunite to form networks.
It is difficult to identify all the afferent vessels for a lymph node because the entire drainage area cannot be easily injected in a single sample, but the number of afferent vessels is certainly greater than the number of efferent vessels.
My investigations into the lymphatic system of dogs have confirmed the general behaviour of the lymph vessels that I reported in cattle (see page 7 of The Lymphatic System of Cattle ). This is particularly true for my finding that, contrary to previous belief, lymph vessels may enter directly into the venous system and that lymph vessels enter the thoracic duct without having passed through a lymph node. The number of vessels that behave in this manner is considerably greater than I assumed after my observations in cattle, and in a large number of cases, these lymph vessels come from outside the thoracic and abdominal cavity, more specifically from organs located at a great distance from the thoracic duct. For example, lymph vessels from certain muscles, including the M. trapezius, M. longissimus dorsi, M. sternohyoideus, M. obliquus capitis caudalis, and M. obliquus externus abdominis, as well as lymph vessels from the thyroid gland and testes, open directly into the thoracic duct and thus drain directly into the venous system. Remarkably often, in one-quarter to one-half, or even up to two-thirds of cases, this was seen for lymph vessels from organs with internal secretion, including the kidneys and adrenal glands, the thyroid gland, and the testes (details,  and Figure 1).
I found that the general behaviour of lymph vessels between cattle and dogs differed in only a single, but very important, aspect: the network formation by larger lymph vessels. Network formation by larger lymph vessels is seldom observed in cattle, though it appears to be quite common in dogs. This is easily demonstrated when, as an example, the drawings of skin lymph vessels of the cow and the dog are compared. The rich network of lymph vessels in the dog makes the exact number of lymph vessels, both those that are associated with blood vessels and those that are not, difficult to count. The number of lymph vessels not associated with blood vessels appears to be considerably less in the dog than in the cow.
All the diagrams of the canine lymphatic system in this publication were illustrated by the artist Georg Münch from Dresden. Mr. Münch carried out this difficult task with great devotion, patience, and artistic understanding, and I am indebted to him for his work.