Part 196

The =vena azygos= resembles that of the horse, and receives at the ninth or tenth thoracic vertebra a =vena hemiazygos=.

Two =jugular veins= are present on each side. The =external jugular vein= is formed by the union of external and internal maxillary veins at the posterior border of the submaxillary gland. The two external jugulars are commonly united by a transverse branch below the cricoid cartilage. Each passes along the neck on the sterno-cephalicus, covered only by the skin and panniculus, dips under the cleido-cervicalis, and joins the internal jugular.

The =external maxillary vein= arises on the lateral nasal region by the junction of the dorsal nasal vein with the angularis oculi. Near the infraorbital foramen it receives the lateral nasal vein, and a little lower the superior labial. In its course along the anterior border of the masseter it receives the vena reflexa, which arises in the pterygo-palatine fossa by the junction of infraorbital, sphenopalatine, and palatine radicles, together with a branch from the cavernous sinus. At the lower border of the mandible it is joined by the inferior labial vein, which receives the buccinator vein. The lingual vein is connected with its fellow by a superficial transverse branch at the insertion of the sterno-hyoidei. The sublingual and submental veins terminate in a variable manner, but often form a common trunk which joins the lingual.

The =internal maxillary vein= arises from the pterygoid plexus, formed chiefly by dorsal lingual, inferior alveolar, deep temporal, pterygoid, and meningeal tributaries. It receives the superior cerebral, auricular, superficial temporal, transverse facial, and masseteric veins, and often a trunk formed by the union of the inferior cerebral and occipital veins.

The =internal jugular vein= results usually from the junction of laryngeal and thyroid veins, but in some cases it is formed by the confluence of the inferior cerebral and occipital veins.

The =brachial= and =radial veins= are satellites of the arteries.

The =ulnar vein= is usually double. It unites below the carpus with a branch of the interosseous vein to form the superficial venous arch.

The =cephalic vein= accompanies the ulnar artery in the forearm and joins the superficial venous arch below.

The =accessory cephalic vein= arises from the union of three dorsal metatarsal veins. It joins the cephalic about the middle of the forearm.

There are three short =volar metacarpal veins= which open into the superficial venous arch. They are formed above the metacarpo-phalangeal joints by the junction of the =volar digital veins=, of which there are two for each of the chief digits. The volar vein of the first digit joins the superficial venous arch.

Each of the chief digits has two =dorsal digital veins=, while the first digit has one.

The =posterior vena cava=, its collateral affluents, and common iliac radicles present no special features of importance.

The =internal iliac vein= corresponds in regard to its tributaries with the branches of the artery, except that it is not divided into dorsal and ventral branches.

The =external iliac=, =femoral=, and =popliteal veins= with their collateral tributaries are satellites of the arteries.

The =anterior tibial vein= is usually double, and the =posterior tibial vein= is very small.

The =internal saphenous vein= is the upward continuation of the internal plantar metatarsal vein. It communicates by a large branch with the dorsal metatarsal vein, and ascends the leg as a satellite of the saphenous artery and its plantar (posterior) branch. The =recurrent tarsal= or =external saphenous vein= is larger. It is formed at the lower part of the leg by the union of dorsal and external plantar metatarsal veins, crosses the outer surface of the leg obliquely upward and backward, ascends behind the gastrocnemius and joins the posterior femoral vein.

The =metatarsal= and =digital veins= resemble in general the corresponding vessels of the fore limb.

THE LYMPH VESSELS AND GLANDS

The =cisterna chyli= is large and fusiform. The =thoracic duct= may be single throughout, but often divides anteriorly into two branches, which may then unite and form a dilatation which receives the left tracheal duct and the vessels from the fore limb. The primitive plexiform arrangement persists in varying degree. The duct terminates in the left brachiocephalic vein.

The =tracheal ducts= accompany the internal jugular veins. The right duct opens into the right brachiocephalic vein.

The =submaxillary lymph glands= (Fig. 486) are commonly three in number on each side. They are situated in the angle between the masseter and the submaxillary salivary gland, in relation to the external maxillary vein and covered only by the skin and panniculus. The number may be reduced to two or one with a corresponding increase in size.

A small round =parotid= or =auricular lymph gland= is situated superficially between the upper part of the posterior border of the masseter and the parotid gland.

The =pharyngeal= (or retropharyngeal) =lymph glands=, one or two in number on each side, lie on the pharynx chiefly under cover of the submaxillary salivary gland.

The =prescapular= or =superficial cervical lymph glands= lie on the serratus magnus at the anterior border of the supraspinatus, embedded in a mass of fat. Usually two or three are present on each side, but there may be only one. They are oval and are about an inch long in a dog of medium size when two are present.

The =axillary lymph gland= lies in a mass of fat on the inner face of the lower part of the teres major. It is discoid.

The =cubital lymph gland= is inconstant. It is situated on the deep face of the long head of the triceps or on the latissimus dorsi.

The =mediastinal lymph glands= are small and quite variable in arrangement. Usually one or two glands are found on the course of the internal thoracic vessels at the second segment of the sternum.

The =bronchial lymph glands= are commonly four in number. The largest is situated in the angle of divergence of the chief bronchi, two lie on the origin of the right apical bronchus, and the fourth is between the aortic arch and the anterior face of the left bronchus. They are commonly pigmented.