Lymphatic System

LYMPHATIC SYSTEM. In anatomy, the lymphatic sys tem comprises the lymphoid or adenoid tissue so plentifully dis tributed about the body, especially in the course of the alimen tary canal (see CONNECTIVE TISSUES), lymph spaces, lymphatic vessels of which the lacteals are modifications, lymphatic glands, haemolymph glands, and the thoracic and right lymphatic ducts by which the lymph (q.v.) finally reaches the veins.

Lymph Spaces.

These are mere spaces lying outside the capillaries and between them and the cells of the tissues they nourish. Usually, they have no special lining, though sometimes there is a layer of endothelial cells like those of the lymphatic and blood vessels. Most of these spaces are very small, but the sub-epicranial space of the scalp, the capsule of Tenon in the orbit, and the retropharyngeal space in the neck, are large adapta tions to allow free movement. Opening from these spaces, and also communicating with the serous membranes by small openings (stomata) are the lymph capillaries (see VASCULAR SYSTEM), which converge to the lymphatic vessels. These resemble veins in having an internal layer of endothelium, a middle unstriped muscular coat, and an external coat of fibrous tissue, though in the smaller vessels the middle coat is wanting. They have numerous endothelial valves, formed of two crescentic segments allowing the lymph to pass toward the root of the neck. When the vessels are engorged these valves are marked by a constric tion, and so the lymphatics have a beaded appearance. The ves sels divide and anastomose freely, and do not, like the veins, increase in calibre as they approach their destination. It is usual to divide the lymphatic vessels into a superficial and a deep set; speaking generally, the superficial ones are found near the course of the superficial veins, while the deeper ones accompany the arteries. The lacteals, so called from the milky appearance of their contents during digestion, are lymphatic vessels which carry the chyle from the intestine; they begin in lymphatic spaces in the villi and round the solitary and agminated glands, and pass into the mesentery, where they enter into mesenteric glands be fore reaching the receptaculum chyli.

Lymphatic Glands.

The lymphatic glands are pink bodies situated in the course of the lymphatic vessels, to which they act as filters. They are generally oval in shape and about the size of a bean, but sometimes, especially in the groin, they form irregular flattened masses tin. long, while, at other times, they are so small as almost to escape notice. They are usually found in groups.

Each gland has a fibrous capsule from which trabeculae pass toward the centre, where they break up and interlace, forming a network, producing a cortical and medullary region for each gland; the intervals are nearly filled by lymphocytes, but close to the trabeculae is a lymph path or sinus, which is only lined by endothelial cells and crossed by the reticular stroma of the gland.

In this region foreign particles carried in the lymph are held. Thus the bronchial glands of city dwellers where there is much soot in the air are black from carbon strained off in its passage from the lungs, while the axillary glands of a tattooed arm are blue. The blood-vessels enter at the hilum, and are distributed along the trabeculae. In addition the lymphatic glands are prob ably one of the sources from which the leucocytes are derived.

The exact position of the various groups of glands is very im portant from a medical point of view, but here it is only possible to give a brief sketch which will be helped by reference to the ac companying diagram. In the head and neck are found occipital and mastoid glands (fig. 1), which drain the back of the scalp; internal maxillary glands, in the zygomatic fossa, draining the orbit, palate, nose and membranes of the brain ; preauricular glands, embedded in the parotid, draining the side of the scalp, pinna, tympanum and lower eyelid, and buccal glands, draining the cheek region. In the neck are the superficial cervical glands along the course of the external jugular vein, draining the sur face of the neck; the submaxillary glands, lying just above the salivary gland of the same name and draining the front of the face and scalp; the submental glands, beneath the chin, draining the lower lip, as well as sometimes the upper, and the front of the tongue; the retropharyngeal glands, draining the naso-pharynx and tympanum; the pretracheal glands, draining the trachea and lower part of the thyroid body; and the deep cervical glands, which are by far the most important and form a great mass close to the internal jugular vein ; they receive afferent vessels from most of the glands already mentioned and so are liable to be affected in any trouble of the head or neck, especially of the deeper parts. Into them the lymphatics of the brain pass directly. The lower part of this mass is sometimes distinguished as a separate group called the supra-clavicular glands, which drain the back of the neck and receive afferents from the occipital and axillary glands. The efferents from the deep cervical glands join to form a com mon vessel known as the jugular lymphatic trunk, and this usually opens into the thoracic duct on the left side and the right lym phatic duct on the right.