Patterns of Lymphatic Drainage in the Adultlaboratory

Home , Brachial lymph nodes, Cervical lymph nodes, Cisterna chyli, Inguinal lymph nodes, Lymph duct

J. Anat. (1971), 109, 3, pp. 369-383 369 With 11 figures Printed in Great Britain Patterns of lymphatic drainage in the adult laboratory rat

NICHOLAS L. TILNEY Department of Surgery, Peter Bent Brigham Hospital, and Harvard Medical School, Boston, Massachusetts (Accepted 27 April 1971)

INTRODUCTION This study was undertaken to define and elucidate patterns of lymphatic drainage in the adult laboratory rat. The incentive for the work arose from investigations into the role of regional lymphatics in the sensitization of the host by skin allografts. It has become clear that the response of rats to antigens, investigated increasingly in the available inbred strains, requires an accurate knowledge of lymphoid anatomy and lymphatic drainage routes. Examinations of the lymphatics of specific body areas of the rat have appeared sporadically in the literature, but descriptions of regional drainage patterns, especially of peripheral sites, are unavailable. Previous investigations by Job (1919), Greene (1935) and Sanders & Florey (1940) have con- centrated primarily upon the location of the lymphoid tissues. Miotti (1965) has stressed visceral drainage, and Higgins (1925) has described the lymphatic system of the newborn rat. A more complete definition of both somatic and visceral lymphatic routes is presented.

MATERIALS AND METHODS One hundred and thirty normal adult rats of both sexes, each weighing between 150 and 300 g, were studied. The animals came from five strains: each inbred - Oxford strains of the albino (AO), hooded (HO), agouti (DA), and F1 hybrid of the HO and DA strains - and 'stock' animals from a closed outbred albino colony. Under ether anaesthesia, the site for cutaneous injection was clipped or a serous cavity entered for visceral injection. The majority of animals were killed by ether overdose and dissected within a few minutes because of the rapidity of uptake of the injected material by lymphoid tissue. Several rats were studied between 48 h and 6 weeks after injection to elucidate possible variations in the distribution of the dye with time. Regional nodes and all secondary drainage sites were examined. Three types of injected material were employed. The location of lymph nodes was demonstrated by the intraperitoneal injection of pontamine sky blue, 10-14 days before dissection. This was administered as a sterile 5 00 solution in distilled water at a dose of 1 ml per 100 g body weight. Macrophages coloured with dye remained in lymph nodes long after surrounding tissues have cleared and allowed the identification of even minute lymphoid aggregations. The cutaneous areas were mapped by intra- dermal injections of 0-05-0d10 ml of colloidal carbon (Pelikan Ink, Guinther Wagner, Germany). This material filled lymphatic channels and readily stained draining lymph nodes. Similar injections of 1311 human serum albumin (Radiochemical Centre, 24 ANA 109 370 NICHOLAS L. TILNEY Amersham, Bucks, England) were used to confirm these patterns of drainage. The regional lymph nodes were removed and their radioactive uptake measured in a gamma scintillation spectrometer (Packard Series 410A). Passive muscular motion was helpful in forcing the dye along peripheral lymphatics. Subserosal injections of hollow viscera or subcapsular injections of solid organs using a no. 30 needle satisfactorily demonstrated visceral lymphatic patterns. The lymph trunks between lymph node groups were visualized by injections of minute volumes of India ink directly into the nodal substance: gentle external pressure on the node caused filling of the efferent lymphatics. Accurate localization of 'lymphatic taps' - the valved junctions of the major lymph ducts with the subclavian veins - was facilitated by total replacement of the circulating blood volume of the animal with saline. Carbon particles could then easily be seen flowing from the lymph ducts into the veins. Lymph node groups have been classified into somatic nodes, which drain the skin and underlying musculature (Table 1), and visceral nodes, which drain primarily the thoracic, abdominal and pelvic organs (Table 2). Somatic nodes generally lie external to skeletal muscle in subcutaneous or areolar tissue, or in fossae between muscle masses. These nodes have been divided functionally into a peripheral and central group. The peripheral group of somatic nodes drains skin and musculo-skeletal sites only. The central group drains similar anatomical areas, but also receives lymphatic channels from peripheral nodes. The nomenclature of the lymph nodes is that of Job (1915) and Sanders & Florey (1940), although modifications have been made to emphasize specific drainage patterns (Fig. 1). 'Lymph trunks' were defined as the large channels connecting various groups of nodes. 'Lymphatic ducts' drain particular groups of nodes and empty directly into the cisterna chyli, or into the subclavian veins through the 'lymphatic taps'. RESULTS The location of lymphoid tissue and distribution of lymphatic channels was con- sistent in individual animals and among the strains of rats used in the study, despite occasional variation in size and number of nodes. Well-defined cutaneous areas were drained by specific groups of nodes (Fig. 2). Nodes within these groups drained distinct sites, although there was usually some overlap between adjacent nodes. Extensive lymphatic plexuses, present throughout the subdermis and in the submucosa of hollow viscera, emptied into constant lymph channels which generally followed veins toward the regional nodes. No peripheral channels entered any body cavity with perforating blood vessels. Junctions between lymphatics and veins other than at the subclavian veins, or direct lymphatico-venous anastomoses within lymph nodes themselves, were not demonstrated. Somatic nodes Head and neck The peripheral lymph nodes draining the head and neck are the superficial cervical, facial and internal jugular groups (Fig. 3). The superficial cervical nodes lie at the upper poles of the submandibular glands and drain the tongue and nasolabial Patterns of lymphatic drainage in the adult laboratory rat 371 lymphatic plexus through channels running between the borders of the digastric and masseter muscles. A small efferent lymphatic curls around the lateral aspect of each submandibular gland to join efferent channels from the facial nodes, or to enter this group directly.

Fig. 1. Lymph node groups in the dissected adult rat. Nodes lying dorsally are demonstrated by reflecting muscles and viscera. Adapted from Sanders & Florey (1940).

The facial nodes lie dorsal to the lower poles of the submandibular glands at the junction of the anterior and posterior facial veins with the external jugular veins. They drain the skin of the head and the ventral aspect and sides of the neck through lymph vessels running with the corresponding facial veins. A large efferent facial trunk loops laterally around each sternocleidomastoid muscle to enter the posterior cervical nodes. As has been noted by Higgins (1925) in studies of newborn rats, a small area of the forehead has minimal lymphatic drainage and presumably repre- 24-2 372 NICHOLAS L. TILNEY sents a lymphatic watershed. Rarely, a tiny posterior auricular lymph node drains the ear, although generally lymphatics with the auricular vein empty into the posterior facial channel.

/Cz Cervical lymph nodes E] Inguinal \9 5 Brachial m Popliteal = Axillary Gluteal

Fig. 2. The cutaneous areas drained by somatic lymph node groups.

Table 1. Somatic lymph nodes of the rat Region Lymph and type node group Location Area drained Efferent drainage Head and neck Peripheral Superficial cervical Upper pole of sub- Nasolabial plexus Central cervical nodes nodes mandibular gland Facial Junction of facial Head and neck Central cervical nodes veins Internal jugular Ventral to brachial Deep cervical Central cervical nodes plexus viscera Central Posterior cervical Dorsal to brachial Peripheral cervical Cervical duct nodes plexus nodes, deep cervical viscera Upper extremity, Trunk Peripheral Brachial Triceps muscle Upper extremities, Axillary nodes nodes shoulders, chest Central Axillary Axilla Upper extrem- Subclavian duct nodes ities, trunk, brachial nodes Hindquarters, Lower extremit y Peripheral Inguinal Flank Thigh, haunches, Axillary nodes nodes scrotum, lateral tail Popliteal Popliteal space Foot, hind leg Lumbar, inguinal nodes Tail Gluteal Sciatic foramen Tail Caudal, lumbar, inguinal, popliteal nodes Patterns of lymphatic drainage in the adult laboratory rat 373 The internal jugular nodes lie close to the posterior cervical nodes, but belong functionally to the peripheral group. They are ventral to each brachial plexus, immediately lateral to the carotid sheath, and drain the pharynx, larynx and the proximal part of the oesophagus through pharyngeal lymphatics running along the surface of the deep cervical muscles. A short lymphatic from each internal jugular node enters the adjacent posterior cervical node directly. The posterior cervical nodes lie dorsal to the brachial plexus on each side. These single central nodes drain all peripheral nodes in the neck and may receive minor tributaries from deep cervical structures. They empty into the posterior aspect of each subclavian vein through large cervical lymph ducts which run into the thorax dorsal to the carotid sheaths.

Anterior facial lymphatic M ms. Nasolabial lymphatic Masseter ma. Superficial cervical nodesDiatcm. Parotid gland Submandibular gland Posterior facial lymphatic Internal jugular node IS I( - ~~~~~Pharyngeal lymphatic Facial nodes /Facial nodes ~~~~~~~~~~~~Facialtrunk Posterior cervical node v J, Brachial plexus Carotid a. -______External jugular v. duct Sternocleidomastoid msavical

Fig. 3. Peripheral and deep cervical lymph nodes and lymphatic channels of the head and neck. The arrows demonstrate the direction of lymph flow. The cervical ducts run dorsal to the carotid sheaths and enter the subclavian veins on either side.

Upper extremity and trunk The peripheral group of brachial nodes lies within a fascial envelope high on each triceps muscle. Usually three in number, they drain the forefoot and front leg, shoulders, neck and upper chest (Fig. 4). The dorsal member of the group drains the radial aspect of forefoot and distal foreleg through the afferent cephalic lymph vessel. This follows the cephalic vein and curves across the lateral aspect of the shoulder to enter the node. The intermediate brachial node drains the shoulder and dorsum of the neck through several small lymphatic branches. The ventral brachial node drains the entire foot, foreleg and anterior chest through the brachial lymphatic. This runs with the brachial vein, but divides to enter the ventral brachial and the most rostral member of the group of axillary lymph nodes. A short radicle from each brachial node forms an efferent trunk which pierces the latissimus dorsi muscle and drains into the most caudal axillary node. The central somatic group of four axillary nodes lies along the chest wall at the 374 NICHOLAS L. TILNEY junction of the lateral thoracic and cutaneous maximus veins. These drain the entire trunk and foreleg, and receive efferent trunks from brachial and inguinal groups (Fig. 5). The caudal axillary node receives a large efferent channel from the inguinal nodes which follows the superficial epigastric vein cephalad along the milk line and drains the nipples and the ventral aspects of chest and abdomen. The two intermediate axillary nodes drain the flanks and back through a major lymph vessel which

Fig. 4 Fig. 5 Fig. 4. Cutaneous lymphatic drainage of individual brachial lymph nodes. Fig. 5. Cutaneous lymphatic drainage of individual axillary lymph nodes. joins them by traversing the panniculus carnosus muscle with the cutaneous maximus vein. A minor branch of this lymphatic continues in the deep dermis to the intermediate brachial node. The rostral axillary node receives the brachial lymph channel from the medial side of the arm and axilla. The large subclavian lymphatic duct runs from this node to enter the thorax with the axillary vein and join the subclavian vein with the other lymphatic taps. Patterns of lymphatic drainage in the adult laboratory rat 375

Hind quarters The paired inguinal nodes are embedded in subcutaneous fat in the flanks near the superficial epigastric veins and drain the gluteal regions, thighs and lower abdomen (Fig. 6). The caudal member of the pair drains the hamstring, gluteal and perineal areas and receives channels which cross the groin from the lateral aspects of the tail, scrotum, or vagina. The rostral member drains the anterior thighs and flanks through several small lymph channels. The large efferent inguinal lymphatic trunk courses cephalad along each nipple line to the axillary chains.

An nguinal iode X, /.

Posterior inguinal nodes

Fig. 6. Cutaneous lymphatic drainage of individual inguinal lymph nodes.

The single popliteal node lies in the lateral aspect of the popliteal space near the superficial muscular vein (Fig. 7). It drains the footpad, foot and hind leg through lymph vessels running with greater and lesser saphenous veins. Efferent popliteal trunks follow each femoral vein to a retroperitoneal lymphatic plexus dorsal to the iliac vessels. The main trunk continues centrally to the iliac nodes, while smaller tributaries travel with the superficial epigastric vessels to the inguinal nodes.

Tail The lymphatic drainage of the tail involves three pathways. Caudal channels along its lateral aspect follow the superior gluteal veins to the gluteal lymph nodes. These single nodes lie deep to the gluteal muscles at each sciatic foramen (Fig. 7). An efferent trunk drains to the lymphatic plexus dorsal to each iliac vein and ultimately to the para-aortic lymph chain. Other vessels from the tail course across the lateral scrotal wall and groin to the inguinal nodes. The ventral surface of the tail is drained separately by a channel which follows the middle sacral vein retroperitoneally through the pelvis to the caudal nodes. A minor efferent lymphatic trunk from each gluteal node runs with the inferior gluteal vein to the popliteal node, which serves as a secondary drainage site for the tail. 376 NICHOLAS L. TILNEY

Flexor caudi longus ms. Catudal lymphatics Caudal v. (branch of superior gluteal v.)

Fig. 7. Dissection of the gluteal area. The gluteal lymph node lies deep to the reflected gluteus maximus muscle. The caudal lymphatic trunk enters the pelvis at the sciatic foramen and runs to the para-aortic lymph chain. A small efferent channel from the gluteal node runs with the inferior gluteal vein to the popliteal node.

Visceral drainage Thorax The major lymphatic structures in the thorax are the thymus gland, and the parathymic and posterior mediastinal lymph nodes. The parathymic nodes are embedded in fat on the lateral aspects of the thymic capsule, and drain the thymus gland, the peritoneal cavity, and the superior surface of the liver and liver capsule through the large internal thoracic lymph channels. These channels collect the effluent from the extensive lymphatic plexus on the pleural surface of the diaphragm and are joined along their course by small radicles from the anterior pericardium and parasternal area (Fig. 8). MacCallum (1903) and Olin & Saldeen (1964) have shown that fluid or particles injected into the peritoneal cavity may pass directly through the fene- strated basement membrane of mesothelial cells on the peritoneal surface of the diaphragm, enter lymphatics between phrenic muscle bundles and empty into the diaphragmatic plexus. The posterior mediastinal nodes lie adjacent to the oesophagus on the right, and to the superior vena cava on the left. The right posterior mediastinal node is usually larger and drains the right pleural space and lung, the base of the heart, and the thoracic portion of the oesophagus. The left node drains the left pleural space, thoracic viscera, and the thymus gland. These nodes receive minor paravertebral lymphatics from the diaphragmatic plexus, and are joined by intercostal lymphatics and hilar radicles from the thoracic organs and pericardium. Although no such anastomoses were found in this study, Kluge & Ongre (1968), using thorotrast, suggested that pericardial channels drain directly into rostral and caudal portions of the thoracic duct. Small paravertebral nodes, present inconstantly behind the Patterns of lymphatic drainage in the adult laboratory rat 377 pulmonary vessels, send efferent channels to the posterior mediastinal node group. The thymus lies in the anterior mediastinum, drained directly by the parathymic nodes. Other thymic lymphatics consistently empty into the left posterior mediastinal node, although no drainage to the opposite node was ever demonstrated by the techniques in this study. The mediastinal lymph ducts, major efferent channels from the thoracic nodes of both sides, empty into the dorsum of the subclavian veins. The ducts from parathymic and posterior mediastinal nodes may merge as a single channel or enter the vein separately. Histological examination of the 'mediastinal organ', mentioned by Sanders & Florey (1940), showed only brown fat.

Table 2. Visceral lymph nodes of the rat Lymph Region node group Location Area drained Efferent drainage Thorax Parathymic Lateral to thymus Peritoneal cavity, Mediastinal duct liver, pericardium, thymus Posterior medi- Paravertebral Thoracic viscera, Mediastinal duct astinal gutter pleural space, pericardium, thymus Paravertebral Dorsal to pul- Diaphragm, thoracic Posterior medi- monary vessels viscera astinal nodes Pelvis and Caudal Median sacral vein Ventral tail, anus, Iliac nodes Retroperi- rectum, gluteal toneum nodes Iliac Aortic bifurcation Pelvic viscera, pop- Renal nodes liteal, gluteal, caudal nodes Para-aortic Para-aortic area Pelvic viscera, Renal nodes popliteal, gluteal, caudal nodes Renal Dorsal to renal Kidneys, supra- Renal duct to veins renals, lumbar cisterna chyli lymphatics External lumbar Retroperitoneal Fat pad, psoas Lumbar lymphatics fat pad muscles, pelvic viscera Abdomen Splenic Splenic vein Splenic capsule and Posterior gastric trabeculae nodes Posterior gastric Gastroduodenal Distal oesophagus, Portal nodes vein stomach, pancreas, splenic node Portal Portal vein Liver, splenic, Portal duct to posterior gastric cisterna chyli nodes Superior Root of Duodenum, small Superior mesenteric mesentery bowel, caecum, mesenteric duct ascending, trans- to cisterna verse colon Inferior Mesentery of Descending and Inferior mesenteric mesenteric descending colon sigmoid colon duct to cisterna 378 NICHOLAS L. TILNEY

-Internal jugular v.

Mediastinal ducts Subclavian v. Parathymic nodes Posterior mediastinal nodes Internal mammary a.+v. Internal mammary lymphatic , vessels (ligated) Sternum

Intercostal lymphatics

- Diaphragm

Fig. 8. The sternum has been reflected to the right and the left anterior ribs removed. The diaphragmatic lymphatic plexus drains into the large internal thoracic (mammary) lymphatics, which enter the parathymic lymph nodes. Mediastinal ducts from the parathymic and posterior mediastinal nodes enter the subclavian veins.

Retroperitoneunm and pelvis The nodes draining retroperitoneal and pelvic organs lie adjacent to the abdominal aorta and inferior vena cava (Fig. 9). The small caudal nodes lie with the median sacral vein at the aortic bifurcation, and drain the rectum, internal anal sphincter and ventral aspect of the tail through caudal channels running along the sacrum. Efferent trunks cross the iliac vessels and enter the iliac nodes. Other pelvic lymphatics, especially from testis or ovary, may enter the caudal group or bypass it and drain to nodes along the para-aortic lymph chain. The large iliac nodes lie along each side of the distal aorta, singly or in pairs. They drain pelvic viscera through extensive lymphatic plexuses around the spermatic or ovarian veins, and along branches of the internal iliac veins. They are important secondary drainage sites for caudal, gluteal and popliteal node groups. Efferent para-aortic trunks lead to the renal nodes through an inconstant pair of para-aortic nodes near the lumbar veins. The renal nodes lie dorsal to each renal vein. They drain the kidneys and suprarenal glands, and ultimately receive lymph from all pelvic and retroperitoneal viscera, hind limbs and tail. The right renal node drains into the cisterna chyli through the renal duct (Fig. 10). The left renal node may empty into the cisterna directly, but more frequently an efferent lymph trunk runs across the great vessels to join the right Patterns of lymphatic drainage in the adult laboratory rat 379 renal node. The cisternal group, an inconstant cluster of minute lymph nodes, lies rostral to the left renal vein. These may drain the suprarenal gland, retroperitoneum or diaphragm, and empty into either renal node. Free communication between both para-aortic lymph trunks is assured by one or more connecting lymphatics which cross the great vessels. Engeset & Tjotta (1960) and Tilney (1970) have shown that dye injected into a pelvic organ, the tail or a hind footpad, is taken up rapidly by all retroperitoneal nodes and drains into the cisterna chyli.

Fig. 9. The retroperitoneal lymph nodes drain the pelvic viscera and the caudal aspect of the body. Channels from the hind limbs enter a lymphatic plexus lying dorsal to the iliac vessels. Efferent lymphatics run to the inguinal lymph nodes as well as to the retroperitoneal lymphatic chain. The renal nodes collect the efiluent from all retroperitoneal nodes and empty directly into the cisterna chyli through the renal lymphatic duct.

The small external lumbar nodes are frequently present in the retroperitoneal fat pads where the lumbar veins course toward the inferior vena cava. They drain the retroperitoneal space and psoas muscles, and act as accessory drainage sites for pelvic viscera. Afferent and efferent lymph channels follow the lumbar veins between these minor nodes and the para-aortic trunk.

Abdominal viscera The regional lymph nodes of the abdominal viscera are divided into three groups which empty separately into the cisterna chyli through lymphatic ducts (Fig. 10). The splenic, posterior gastric and portal nodes draining the upper abdominal organs, the superior mesenteric chain draining the bowel, and the interior mesenteric chain draining the descending colon, are considered as distinct functional entities. 380 NICHOLAS L. TILNEY The small splenic node drains the splenic capsule and fibrous trabeculae through lymphatics running with the four hilar veins. Its efferent channel travels centrally with the splenic vein to the posterior gastric node. This node, single or paired, lies adjacent to the gastroduodenal vein and drains the distal part of the oesophagus, the stomach and the pancreas through lymphatics following visceral veins. A short efferent channel enters the two portal nodes, one on either side of the portal vein. Lymph from the inferior aspect of each liver lobe and liver capsule drains to its corresponding node through periportal lymphatics. The portal node group empties into the left side of the cisterna chyli through the portal lymph duct, a large channel running obliquely across the great vessels.

Thoracic duct Aorta Portal v.l\ > Prenalnduct Superior mesenteric duct Superior mesenteric v. Portal duct

Posterior gastric nodes Splenic node

Left renal node

Inferior mesenteric duct Inferior mesenteric v. Inferior mesenteric node Right renal node Caccal node Superior mesenteric nodes Cisterna chyli Para-aortic trunk Fig. 10. Lymphatics of the abdominal viscera drain into three lymph ducts which enter the cisterna chyli. The portal duct drains the nodes from the upper abdominal organs, the superior mesenteric duct drains the small bowel and the ascending and transverse colon, while the inferior mesenteric duct drains the descending and sigmoid colon. The arrows demonstrate the direction of lymph flow.

The large aggregate of superior mesenteric lymph nodes extends across the base of the mesentery and drains the duodenum, small bowel, ascending and transverse colon. The lymphatic plexus present throughout the submucosa of the bowel empties into individual nodes in the chain through lymph vessels along segmental branches of the superior mesenteric vein. The superior mesenteric duct follows the corresponding vein and collects the effluent from each node in the chain. It swings dorsally to enter the right side of the cisterna chyli as the superior mesenteric and splenic veins sweep cephalad to the portal vein. Dye injected into the bowel wall always passed through a node and never entered the mesenteric duct directly. Although Peyer's patches became stained following systemic injection of pontamine sky blue, no efferent lymphatics were demonstrated after direct injection with colloidal carbon. B. J. Roser (1969, personal communication), using carbon and 185W techniques, Patterns of lymphatic drainage in the adult laboratory rat 381 has noted minute efferent channels running from Peyer's patches to regional mesenteric nodes. The inferior mesenteric node lies in the mesentery of the descending colon where the inferior mesenteric vein crosses the great vessels to join the superior mesenteric vein. It drains the rectum and sigmoid colon through a lymph channel coursing beside the vein. The efferent inferior mesenteric duct enters the cisterna on the left side.

Internal jugular node Cervical duct Posterior cervical node Subclavian v. Lymphatic taps Subclavian duct Mediastinal ducts Right superior vena cava

Parathymic nodes cava V.

Inferior vena cava

chyli Y/) Fig. 11. The cisterna lies dorsal to the great vessels and forms the thoracic duct. This duct runs on the right of the great vessels in the thorax but crosses dorsal to them in the neck to enter the left subclavian vein. The cervical, subclavian and mediastinal lymphatic ducts empty into the subclavian veins through their lymphatic taps. Lymphatic drainage of the omentum was not demonstrated in this study. Injected particulate dye was presumably ingested by macrophages which gradually migrated to omental 'milk spots' where they remained for prolonged periods. Casparis (1918) and Simer (1934) have described lymphatics in the fixed omentum of the rat, although none has been demonstrated in vivo. Cisterna chyli and thoracic duct The cisterna chyli lies dorsal to the aorta and vena cava at the level of the renal veins, and narrows within a few millimetres to form the thoracic duct (Fig. 11). The duct runs on the left of the abdominal aorta but crosses deep to the great vessels at 382 NICHOLAS L. TILNEY the diaphragm to continue its thoracic course to the right of the oesophagus. It swings dorsal to the carotid arteries and trachea at the level of the clavicles and empties into the left subclavian vein with the other left lymphatic taps.

DISCUSSION Particulate material such as colloidal carbon, when injected intradermally or subserosally, readily fills lymphatic channels and drains to regional lymph nodes. This reinforces the observations of Hudack & McMaster (1932), who demonstrated that local cutaneous trauma, however mild, causes an immediate increase in lymphatic permeability which permits the entrance of large molecules. Similarly, labelled albumin rapidly enters lymphatics and localizes in lymph nodes. The high concentra- tions of this antigen in the regional lymphoid tissue make it an accurate and effective means of confirming studies with carbon. The location of lymphoid tissue was invariable among the animals examined, although the numbers of lymph nodes in particular node groups varied slightly. This may be dependent in part upon the age of the animal. Fewer nodes were described by Higgins (1925) in studies of the lymphoid system of the newborn rat, and distinct morphological differences were found by Andrew & Andrew (1948) between the lymphoid tissues of young and old animals. Lymphatic channels were constant and often followed adjacent veins. All lymphatics ultimately drained to the lymphatic taps which emptied into the subclavian veins. Anatomical variations sometimes occurred at these major lymphatico-venous junctions, and were also noted by Job (1915) in previous studies on the rat. Somatic areas especially subject to environmental hazards possessed a diffuse regional drainage patttern. Mucocutaneous junctions and the footpads drained to at least two separate nodal groups. The extensive lymphatic supply of the tail has been delineated directly in this study and radiologically by Engeset & Tjotta (1960). Whether lymphatics from a particular site may bypass regional nodes and enter major lymph trunks directly has been debated in the literature. Yoffey & Courtice (1956) were unable to document a complete lymphatic bypass of every node draining a structure, but Engeset (1959) has shown that lymphatics from the testes of rats and dogs do not necessarily enter any nodes on their course to the bloodstream. In the present study lymphatic drainage of the testis occasionally bypassed one or more node groups, but inevitably entered nodes which themselves drained into the cisterna chyli. SUMMARY Accurate knowledge of lymphoid anatomy and lymphatic routes in the laboratory rat has become increasingly important as this animal is used more frequently in biological investigations. Because of the unavailability of precise descriptions, lymphatic drainage patterns were studied in the adult rat using multiple injections of colloidal carbon and 1311 human serum albumin. The regional lymph drainage of the skin is described and visceral lymphatic anatomy defined. The anatomical constancy of lymphoid tissue and lymphatic channels between individual animals and between the various strains examined is stressed. Patterns of lymphatic drainage in the adult laboratory rat 383 I should like to thank Professor J. L. Gowans and Dr B. J. Roser of the Sir William Dunn School of Pathology, University of Oxford, and Professor D. W. Fawcett and Dr E. A. Edwards of the Department of Anatomy, Harvard Medical School, for their encouragement and support. The illustrations were drawn by Miss C. Court, Department of Human Anatomy, University of Oxford, and were corrected and lettered by Miss T. Holden, Peter Bent Brigham Hospital, Boston, Massachusetts, U.S.A. The study was performed while the author was a G. G. Peters Travelling Fellow at the Sir William Dunn School of Pathology, University of Oxford.

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