Part 65

3. The =splenic vein= (V. lienalis) is the very large satellite of the splenic artery. It is formed by the union of two radicles at the base of the spleen. On leaving the hilus of the spleen it passes inward between the anterior pole of the left kidney and the saccus cæcus of the stomach and above the left end of the pancreas, receives commonly the =posterior gastric vein= (V. gastrica caudalis), and unites with the anterior mesenteric at the posterior border of the pancreas.

The collateral tributaries of the portal veins are as follows:

(1) =Pancreatic veins= (Rami pancreatici).

(2) The =gastro-duodenal vein= (V. gastroduodenalis) corresponds mainly to the extrahepatic branches of the hepatic artery.

(3) The =anterior gastric vein= (V. gastrica cranialis) joins the portal at the portal fissure.

THE COMMON ILIAC VEINS (Fig. 456)

These (V. iliacæ communes) are two very large but short trunks which result from the union of the internal and external iliac veins of each side at the sacro-iliac articulation. The left one is the longer and crosses obliquely over the terminal part of the aorta. The chief tributaries of each are as follows:

1. The last =lumbar vein=.

2. The =circumflex iliac veins= (Vv. circumflexæ ilium profundæ) are the two satellites of each corresponding artery, on either side of which they are placed. They may open directly into the posterior vena cava or into the external iliac vein.

3. The =ilio-lumbar vein= (V. iliolumbalis) may open into the common iliac, the external iliac, or the internal iliac vein.

THE INTERNAL ILIAC VEINS

The =internal iliac= or =hypogastric veins= (Vv. hypogastricæ), right and left, are usually formed by the confluence of lateral sacral, gluteal, and internal pudic veins. The obturator vein may open into them also. They are short trunks and are smaller than the external iliac veins. The tributaries correspond in general to the branches of the three arteries of like names. The internal pudic veins receive affluents from the venous plexuses of the prepuce and penis in the male; of the mammary gland, vulva, vagina, and vestibular bulb in the female.

THE VEINS OF THE PELVIC LIMB

The =external iliac vein= (V. iliaca externa) (Figs. 450, 451) lies behind the corresponding artery at the brim of the pelvis. It is the upward continuation of the femoral vein, and unites at the sacro-iliac joint with the internal iliac to form the common iliac vein. Its tributaries are as follows:

1. The =obturator vein= (V. obturatoria) is a satellite of the artery and usually opens into the external iliac at the insertion of the psoas minor. Its radicles anastomose with those of the internal and external pudic veins (Figs. 451, 455).

2. The =iliaco-femoral= or =external circumflex veins= (Vv. circumflexæ femoris laterales) are the two satellites of the homonymous artery. They open a little higher than the obturator.

The =femoral vein= (Figs. 450, 451) lies behind the artery in the upper part of the thigh, external to it lower down. Its chief tributaries are:

1. A very large but short trunk formed by the union of the deep femoral and the external pudic vein. The =deep femoral vein= (V. profunda femoris) corresponds otherwise to the artery. The =external pudic vein= (V. pudenda externa) arises chiefly from a rich plexus of large veins situated above and along the sides of the penis and prepuce in the male, the mammary glands in the female. It passes through a foramen in the anterior part of the tendon of origin of the gracilis and runs outward in the subpubic groove behind the pectineus to unite with the deep femoral vein. The right and left veins are connected by a large transverse anastomosis and each has a large connection with the obturator vein. Each receives the =posterior abdominal vein= (V. epigastrica caudalis) which accompanies the artery of like name. The =subcutaneous abdominal vein= (V. abdominalis subcutanea) arises in the skin and panniculus of the ventral abdominal wall, anastomoses with the internal and external thoracic and deep abdominal veins, and joins the external pudic or posterior abdominal vein.

A small vein accompanies the external pudic artery in the inguinal canal.

2. The =anterior femoral vein= (V. femoris cranialis) accompanies the artery.

3. The =saphenous vein= (V. saphena) (Figs. 451, 458, 460) arises at the inner side of the flexion surface of the tarsus as the upward continuation of the internal metatarsal vein. Its course is distinctly visible. It ascends on the subcutaneous surface of the tibia and the popliteus muscle, inclosed between layers of the deep fascia, inclines a little backward to the proximal part of the leg, then deviates slightly forward, runs upward on the gracilis, passes between that muscle and the sartorius, and joins the femoral or the external pudic vein.[173] On the upper part of the capsule of the hock joint it forms an arch with the anterior tibial vein. The vein has numerous valves. The satellite artery is relatively small and lies in front of the vein as far as the junction with the recurrent tibial vein, which it accompanies on the leg. It receives the =recurrent tibial vein= (V. recurrens tibialis)[174] at the proximal fourth of the leg. This vessel arises at the inner surface of the tarsus and forms an arch with the posterior tibial vein at the level of the tuber calcis. It ascends in the furrow in front of the gastrocnemius tendon, inclines forward at the proximal third of the leg, and joins the saphenous vein at an acute angle. It has numerous valves. A smaller vein from the anterior face of the metatarsus joins the saphenous at the hock.

4. =Muscular branches= which correspond to the arteries.

5. The =posterior femoral= or =femoro-popliteal= vein (V. femoris caudalis) is a satellite of the artery. It receives the =recurrent tarsal= or =external saphenous vein= (V. tarsea recurrens), which arises at the outer side of the hock, ascends on the deep fascia of the external surface of the leg in front of the tendo Achillis, passes between the biceps femoris and semitendinosus, and joins the posterior femoral vein. It is connected with the recurrent tibial vein by a large anastomotic branch which crosses in front of the tuber calcis. Usually a branch from it ascends along the great sciatic nerve and anastomoses with the obturator vein.

The =popliteal vein= (V. poplitea) lies along the inner side of the artery (Fig. 459). It is formed by the confluence of anterior and posterior tibial veins.

Two =anterior tibial veins= (Vv. tibiales anteriores) usually accompany the artery of like name; the outer vein is much the larger. In other cases there is a single large vein in the proximal part of the leg, two lower down. They arise from a number of anastomosing radicles on the front of the capsule of the hock joint, chiefly as the continuation of the perforating tarsal vein. The origin of the chief vein is connected with the saphenous by a large anastomotic branch.

The =posterior tibial vein= (V. tibialis posterior) is commonly double (Fig. 458). It arises at the level of the tuber calcis, where it has a communication with the recurrent tibial vein. It is a satellite of the artery.

The =internal= or =great metatarsal vein= (V. metatarsea dorsalis medialis) (Figs. 458, 460) arises from the venous arch above the sesamoids at the fetlock, but is practically the upward continuation of the internal digital vein. It ascends along the inner border of the deep flexor tendon, then in the groove on the inner aspect of the proximal part of the large metatarsal bone to the capsule of the hock joint, and is continued by the saphenous vein.

The =external metatarsal vein= (V. metatarsea plantaris lateralis) arises from the venous arch above the fetlock, but appears to be the upward continuation of the external digital vein. It ascends along the outer border of the deep flexor tendon in front of the plantar nerve, and is connected with the deep metatarsal vein at the proximal part of the metatarsus by a transverse branch. It then passes upward along the deep flexor tendon in relation to the plantar nerves and the inner tarsal artery and is continued by the recurrent tibial vein.

The =deep metatarsal vein= (V. metatarsea plantaris medialis) arises from the plantar venous arch, passes forward between the branches of the suspensory ligament, and ascends on the posterior face of the large metatarsal bone. At the proximal end of the metatarsus it is connected with the external metatarsal vein by a transverse branch. It then passes (as the perforating tarsal vein) through the vascular canal of the tarsus and forms the chief radicle of the anterior tibial vein.

The =plantar venous arch= (Arcus venosus plantaris) and the digital veins are arranged like those of the thoracic limb.

The Lymphatic System

The =lymphatic system= (Systema lymphaticum) is subsidiary to the venous part of the circulatory system, from which it arises in the embryo. It consists of the lymph vessels and glands.

The =lymph vessels= (Vasa lymphatica) contain a colorless fluid, the lymph, which contains numerous lymphocytes.[175] They resemble the veins in structure but have thinner walls and are provided with more numerous valves. The vessels are sacculated opposite the segments of the valves and have a characteristic beaded appearance when distended. The collecting lymph vessels do not usually form rich plexuses, as veins often do, their branching is more limited and less tree-like than that of the blood-vessels, and their caliber therefore increases less from the periphery toward their termination. All of the lymph is ultimately carried into the venous system by two trunks, the =thoracic duct= and the =right lymphatic duct=. Almost all of the lymph passes through at least one group of lymph glands before entering the blood-vascular system.

The =lymph glands= or =nodes= (Lymphoglandulæ) are intercalated in the course of the lymph vessels. They vary widely in size, some being microscopic, others several inches in length. In form they may be globular, ovoid and flattened, elongated, or irregular. In certain situations they are aggregated into groups, and a knowledge of the position of these and the territory drained into them is important. It is convenient, when possible, to indicate their position with regard to arteries on the course of which they are placed. In color they are usually gray or yellowish-brown in the dead subject, pink or reddish-brown during life, but this varies according to their position and functional state. The bronchial glands are often blackened by infiltration with carbon. The mesenteric glands are creamy or white while the chyle is passing through, but pink at other times. Vessels which carry lymph to a gland are called =afferent=; the =efferent= vessels which convey it away are larger and fewer. Each gland has a depression, the =hilus=, at which the blood-vessels enter and the efferent lymph vessels emerge.

=Lymph nodules= or =follicles= (Noduli lymphatici) are minute masses of lymphoid tissue which occur in certain mucous membranes. They may be solitary, as in the solitary glands of the intestine, or aggregated into masses or patches, as in the tonsils and the so-called Peyer’s patches.

The =lymph nodule= or =follicle= is the unit of structure of the lymph gland. It consists essentially of an artery surrounded by a =reticulum= of connective tissue, the meshes of which contain numerous =lymphocytes=. Surrounding this is a rich plexus of lymph vessels, forming the so-called =sinus=, inclosed in some cases by a fibrous capsule. The gland consists of a mass of follicles, inclosed in a fibrous =capsule=, from which =trabeculæ= pass in and unite the follicles. Beneath the capsule is the =peripheral sinus=, which consists of a very rich plexus of lymph vessels; to this the afferent vessels pass at various points of the surface. In the =cortical substance= the cells are in rounded masses, the cortical nodules, while in the =medullary substance= they lie around the arteries, forming the so-called medullary cords. The =medulla= is redder than the cortex, since it is more vascular; it contains the =central lymph sinuses=, which have a similar structure to the peripheral sinus.

The =hæmolymph glands= differ from the lymph glands in color and structure. They are of a deep red color, which is due to the high vascularity of the cortical substance. The peripheral sinuses especially are greatly developed and contain numerous red blood-cells. There is no clear division into cortical and medullary substance, and the trabeculæ contain smooth muscle-cells. Some have afferent and efferent lymph vessels and others do not. They resemble the spleen in some respects, but their significance is not yet clear. They are numerous in the ox and sheep, much fewer in the dog, and apparently are absent in the horse. They occur along the course of the aorta, in the perineal fat, at the portal fissure, and with the gastric and mesenteric lymph glands. In the ox they are also found under the trapezius muscle, under the skin of the upper part of the flank, and in other places less constantly.

The =tissue= or =lymph-spaces= are interstices of varying size between cells or in the meshes of connective tissue. They contain a fluid derived from the blood-plasma, which is usually called =lymph=. They are drained by the veins and lymph vessels. The large serous sacs are often included in this category.

The exact relationship between the lymphatic capillaries and the tissue spaces is still a matter of controversy. It is held by some that the lymph vessels are in direct communication with the tissue spaces, while others maintain that the lymphatics are complete closed tubes. Communication between the spaces and vessels is in general very free. Mall has shown that granules injected into the hepatic artery are returned by the lymphatics as well as by the veins, and intramuscular injections will enter the lymph vessels of the tendon in spite of the absence of lymphatics in muscle.

LYMPHATIC SYSTEM OF THE HORSE

THE THORACIC DUCT (Figs. 428, 429)

The =thoracic duct= (Ductus thoracicus) is the chief collecting trunk of the lymphatic system. It begins as an elongated irregular dilatation, the =cisterna= or =receptaculum chyli=, which is situated between the right side of the aorta and the right crus of the diaphragm at the first and second lumbar vertebræ. The duct enters the thorax through the hiatus aorticus and runs forward on the right of the median plane between the vena azygos and the aorta, covered by the pleura. At the sixth or seventh thoracic vertebra it inclines somewhat ventrally, crosses obliquely over the left face of the œsophagus, and passes forward on the left side of the trachea to the inlet of the thorax. The extrathoracic terminal part passes downward and forward a variable distance (3 to 4 cm.) on the deep face of the left scalenus muscle, bends inward and backward under the bicarotid trunk, and opens into the upper part of the origin of the anterior vena cava just behind the angle of junction of the jugular veins. The terminal bend is ampullate and sometimes divides into two very short branches which open close together.

Since the duct develops from a plexus of ducts in the embryo, considerable variation from the more usual course occurs. There is often a left duct which arises at the cisterna or at a variable point from the right duct, runs across the left intercostal arteries parallel to the latter, and unites with it over the base of the heart or further forward. The two are connected by cross-branches. In some cases the left duct is the larger, and there may indeed be none on the right side. Other variations are common.

The chief tributaries of the thoracic duct are as follows:

1. The two =lumbar trunks= (Trunci lumbales) are formed by the confluence of the efferent ducts of the lumbar glands, and commonly unite with each other and with the posterior intestinal trunk before opening into the cistern.

2. The =intestinal trunks= (Trunci intestinales), two or three in number, receive the efferents of the lymph glands of the intestine, stomach, liver, and spleen.[176]

In its course through the thorax the thoracic duct receives efferents from the intercostal, mediastinal, and bronchial glands. At the thoracic inlet it is joined by ducts from the prepectoral and right axillary glands, and by the left tracheal duct. The duct is provided with several pairs of valves; the best developed are at its termination.

THE RIGHT LYMPHATIC DUCT

This vessel (Ductus lymphaticus dexter)—when present—collects the lymph from the right side of the head, neck, and thorax, and from the right thoracic limb. It is most frequently absent, being represented by a number of short ducts which terminate in the thoracic duct, the right jugulo-brachial junction, or the origin of the anterior vena cava. When present in its typical form, it results from the confluence of efferent ducts from the right axillary and prepectoral lymph glands with the right tracheal duct. It lies on the deep face of the scalenus muscle above the terminal part of the right jugular vein. It is more or less ampullate and usually opens into the anterior vena cava to the right of the thoracic duct. It may be connected with the latter by considerable anastomoses and may join it.

The duct is very variable in form and in regard to its afferents. Often it is a very short, irregular, and bulbous trunk; in some cases it is about an inch and a half (ca. 3 to 4 cm.) in length and receives the tracheal duct at its terminal bend. The lymphatico-venous connections here need further study.

THE LYMPH GLANDS AND VESSELS OF THE HEAD AND NECK

1. The =submaxillary lymph glands= (Lg. submaxillares) (Figs. 437, 462) are arranged in two elongated groups in the submaxillary space along each side of the omo-hyoid muscles. The two groups are in apposition in front of the insertion of these muscles and diverge posteriorly in the form of a =V=, extending backward about four or five inches (ca. 10 to 12 cm.). They are covered by the skin and a thin layer of fascia and panniculus, and are therefore palpable. Anteriorly they are firmly attached to the mylo-hyoidei, but otherwise they are rather movable in the normal state. Each group is related externally to the external maxillary artery and the anterior belly of the digastricus, below to the external maxillary vein, and above to the lingual and sublingual veins.

They receive =afferent vessels= from the lips, nostrils, nasal region, cheeks, the anterior part of the tongue, the jaws, the floor of the mouth, and the greater part of the hard palate and nasal cavity. The =efferent vessels= pass to the anterior cervical and pharyngeal glands.

The superficial lymph vessels of the face converge to twelve to fifteen trunks which turn around the lower border of the jaw with the facial vessels. Those of the lips form plexuses at the commissures. The nasal mucous membrane is richly supplied with lymph vessels which accompany the veins; posteriorly they communicate with the subdural and subarachnoid spaces and send efferents to the pharyngeal and anterior cervical glands.

2. The =pharyngeal lymph glands= (Figs. 436, 437, 569) may consist of two groups. One lies on the lateral surface of the pharynx along the course of the external carotid artery. These glands are related externally to the stylo-maxillaris and digastricus and often to the submaxillary gland also, above to the guttural pouch. Other glands (Lg. retropharyngeales) are commonly found on the guttural pouch along the course of the internal carotid artery. They lie below the artery and are covered by the aponeurosis of the mastoido-humeralis and the cervical end of the submaxillary gland.

They receive afferent vessels from the cranium, the posterior part of the tongue, the soft palate, pharynx, guttural pouch, larynx, posterior part of the nasal cavity, and efferents from the submaxillary glands.

3. The =anterior cervical lymph glands= (Lg. cervicales craniales) are situated chiefly along the course of the common carotid artery in the vicinity of the thyroid gland, under cover of the cervical angle of the parotid gland. Some occur between the thyroid and the submaxillary salivary gland, others above and partly upon the thyroid. They are related deeply to the posterior part of the larynx, the trachea, the thyroid gland, and the œsophagus; below to the external maxillary vein and the outer border of the omo-hyoideus.[177]

These glands are variable. Often there are none in front of the thyroid and the group may extend back a considerable distance along the course of the carotid artery.

Their afferents are deep lymph vessels from the head, the pharynx, larynx, guttural pouch, and thyroid gland, and efferents from the submaxillary and pharyngeal glands. Their efferent vessels go to the middle and posterior cervical glands.

4. The =middle cervical lymph glands= (Lg. cervicales mediæ) form an inconstant group situated a little in front of the middle of the neck on the trachea below the carotid artery. The group is usually small and in some cases is absent, being replaced by a number of glands occurring at intervals along the course of the carotid artery. In other subjects the group consists of several glands of considerable size. They are intercalated in the course of the tracheal lymph ducts.

5. The =posterior cervical= or =prepectoral lymph glands= (Lg. cervicales caudales) form a large group below the trachea at the entrance to the thorax (Fig. 466). They occupy the interstices between the vessels and muscles and extend forward a variable distance on the ventral aspect of the trachea. They are covered by the panniculus and sterno-cephalicus. Their afferent vessels come from the head, neck, thorax, and thoracic limb. They receive efferent ducts of the anterior and middle cervical, prescapular, and axillary glands. Their efferents go to the thoracic duct on the left, to the right lymphatic duct on the right, or open directly into the vena cava.

6. The =prescapular= or =superficial cervical lymph glands= (Lg. cervicales superficiales) lie on the anterior border of the anterior deep pectoral muscle, in relation to the omo-hyoideus internally and the mastoido-humeralis externally (Figs. 431, 441, 466). They are on the course of the ascending branch of the inferior cervical artery. They receive afferents from the neck, breast, shoulder, and arm. Their efferents pass to the prepectoral glands.

The =tracheal ducts=, right and left (Ductus trachealis dexter, sinister), are collecting trunks for the lymph of the head and neck. They lie on the trachea in relation to the carotid arteries. The right one goes to the prepectoral glands or to the right lymphatic duct, the left one to the terminal part of the thoracic duct.

THE LYMPH GLANDS AND VESSELS OF THE THORAX

1. The =intercostal lymph glands= (Lg. intercostales) are small and are situated at the sides of the bodies of the thoracic vertebræ, in series corresponding to the intercostal spaces. They receive afferents from the vertebral canal, the spinal muscles, the diaphragm, intercostal muscles, and pleura. The efferent vessels go to the thoracic duct.

In the young subject these glands are more numerous than in the adult, and there are also glands along the dorsal face of the thoracic aorta which seem to disappear later.