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capillaries start as closed tubules, and the endothelial walls of these tubules play an active part (secretory) in determining when water and other substances shall be admitted into the capillary and further determine the quantity of such discharge. Apparently, too, these cells are specifically excited when the tissue is thrown into activity, the exciting substance being a metabolite from the active tissue. Leucocytes also are capable of passing through or between the endothelial cells of the lymph Ca, p1113, ]'y - G.

LYMPHATIC SYSTEM. In anatomy, the lymphatic system (Lat. lympha, clear water) comprises the lymphoid or adenoid tissue so plentifully distributed about the body, especially in the course of the alimentary canal (see CONNECTIVE TISSUES), lymph spaces, lymphatic vessels of which the lacteals are modihcations, lymphatic glands, haemolymph glands, and the thoracic and right lymphatic ducts by which the lymph (q.v.) finally reaches the veins.

Lymph spaces are mere spaces in the connective tissue, which usually 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 sometimes, as in the case of the sub-epicranial space of the scalp, the capsule of T enon in the orbit, and the retro pharyngeal space in the neck, they are large and are adaptations to allow free movement. Opening from these spaces, and also communicating with the serous membranes by small openings called stomata,1 are the lymph capillaries (see VASCULAR SYSTEM), which converge to the lymphatic vessels. These resemble veins in having an internal layer of endothelitun, 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 constriction, and so the lymphatics have a beaded appearance. The vessels divide and anastomose very freely, and for this reason they 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. Probably any single drop of lymph passes sooner or later through one or more lymphatic glands, and so those vessels which are approaching a gland are called ajerent, while those leaving are spoken of as ejerent lymphatics. The lacteals are special 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 come in contact with a large number of mesenteric glands before reaching the receptaculum chyli. 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 2 in. 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, and in this way a cortical and medullary region for eac gland is distinguished; the intervals are nearly hlled by lymphoid tissue, but close to the trabeculae is a lymph path or sinus, which is only crossed by the reticular stroma of the lymphoid tissue, and this probably acts as a mechanical sieve, entangling foreign particles; as an example of this the bronchial glands are black from carbon strained off in its passage from the lungs, while the axillary glands in a tattooed arm are blue. The blood-vessels enter at one spot, the hilum, and are distributed along the trabeculae. In addition to their function as filters the lymphatic glands are probably one of the sources from which the leukocytes are derived. The exact position of the various groups of glands is very important 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 accompanying diagram. In the head are found occipital and mastoid glands (fig. I, 5), which drain the back of the scalp; internal maxillary It has recently been stated that stomata do not exist in the pentoneum.

glands, in the zygomatic fossa, draining the orbit, palate, nose and membranes of the brain; preauricular glands (fig. I, a), 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 (fig. I, 1), along the course of the external jugular vein, draining the surface of the neck; the sub maxillary glands (fig. I, 6), lying just above the salivary gland of the same name and draining the front of the face and scalp; the sub mental glands (fig. I, 6), beneath the chin, draining the lower lip, as well as sometimes the upper, and the front of the tongue; the retro pharyngeal 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 roup

called the 'supra-clavicular glands, which drain the back of the neck

and receive afferent from the

occipital and axillary lands. The

efferent from the cieep cervical

glands join to form a common

vessel known as the jugular lymphatic trunk, and this usually opens

into the thoracic duct on the left

side and the right lymphatic duct

on the right.

In the thorax are found intercostal

glands (fig. 2, I.), near the vertebral column draining the back of the

thoracic walls and pleura; internal

mammary glands, draining the front

of the same parts as well as the

inner part of the breast and the

upper part of the abdominal wall;

diaphragmatic glands, draining that

structure and the convex surface

of the liver; anterior, middle, posterior and superior mediastinal

glands, drainin the contents of

those cavities. The bronchial glands, draining the lungs, have already

been referred to.

In the abdomen and pelvis the

glands are usually grouped round

the large arteries and are divided

into visceral and parietal. Among

the visceral are the gastric glands, draining the stomach (these are

divided into coronary, sub pyloric

and retro pyloric groups); the splenic glands at the hilum of the spleen,

draining that organ, the tail of the pancreas and the fundus of the

stomach; the hepatic glands in the 5, Submaxillary. small omentum, draining the lower 6, Submental. § ', lnfraclavicular.

surface and deep parts of the liver; the pancreatic glands, behind the 17, Anterior axillary. 9, Supratrochlear.

lesser sac of the peritoneum, draining the head and body of the L, Antecubital. pancreas, the superior mesenteric

glands; from one to two hundred

in number, lying in the mesentery

and receiving the lacteals; the ileo-caecal glands, draining the caecum, one of whic is known as the appendicular gland and drains the vermiform appendix and right ovary; the colic glands along the right and middle colic arteries, draining the ascending and transverse colon; the inferior mesenteric glands in the course of that artery, draining the descending iliac and pelvic colons; the rectal glands, behind the rectum, draining its upper part.

Among the parietal glands are the external iliac glands, divided into a lateral and mesial set (see fig. 2, E.I.), and receiving the inguinal efferent vessels and lymphatics from the bladder, prostate, cervix uteri, upper part of the vagina, glans penis vel clitorides and urethra. The supra and infra-umbilical glands receive the deep lymphatics of the abdominal wall, the former communicating with the liver, the latter with the bladder. From the latter, vessels pass to the epigastric gland lying in front of the termination of the external iliac artery. The internal iliac glands (fig. 2, I. I.) are situated close to the branches of this artery and drain the rectum, vagina, prostate, urethra, buttock and perineum. Common iliac glands (fig. 2, C.I.) lie around that artery and receive afferent from the external and internal iliac glands as well as a few from the pelvic viscera? The K, Inguinal.

l, Superficial femoral.

2 For further details of the pelvic glands see “ Seventh Report the Committee of Collective Investigation, " fourn. Anat. and Phys. xxxli. 164.