Throughout their course are found numerous semilunar valves, looking toward the larger vessels, formed by a folding of the inner coat and strengthened by connective tissue.

Lymphatic Glands consist of an external fibrous covering, from the inner surface of which partitions of fibrous tissue, the trabecula, pass into the substance of the gland, forming a stroma or network, in the meshes of which are found the true lymph corpuscles.

The lymphatics which enter the gland are called the afferent vessels; those which leave it, the efferent vessels.

The Thoracic Duct is the general trunk of the lymphatic system, into which the vessels of the lower extremities, of the abdominal organs, of the left side of the head and left arm empty their contents. It is about twenty inches in length, arises in the abdomen, opposite the third lumbar vertebra, by a dilatation, the receptaculum chyli; ascends along the vertebral column to the seventh cervical vertebra, and terminates in the venous system at the junction of the internal jugular and subclavian veins on the left side. The lymphatics of the right side of the head, of the right arm and the right side of the thorax, terminate in the right thoracic duct, about one inch in length, which joins the venous system at the junction of the internal jugular and subclavian on the right side.

Lymph is a clear, transparent fluid, slightly alkaline, having a saline taste and a specific gravity of 1.022. It is found in the lymphatic vessels throughout the body.

Lymph contains a number of corpuscles (the leucocytes) resembling the white corpuscles of the blood, which increase in number as it passes through the lymphatic glands. They are about of an inch in diameter and somewhat granular; they are discharged into the blood, but their function is obscure. When withdrawn from the vessels, lymph undergoes spontaneous coagulation, separating into serum and clot, as in the case of the blood.

Origin of Lymph. Lymph is undoubtedly a transudation from the capillary blood vessels, occurring during the process of nutrition, and is

identical, for the most part, with the liquor sanguinis, or plasma. As new material is constantly exuded, the old is absorbed by the lymphatics, and returned again to the circulation.

Excrementitious matters, as urea, cholesterine, etc., are also taken up from the tissues by the lymphatics and emptied into the blood.

Diagram showing the course of the lacteals through the mesentery, and their termination in the thoracic duct. The course of the thoracic duct and its termination at the junction of the internal jugular and subclavian veins.

The total quantity of lymph poured into the thoracic duct in 24 hours has been estimated at 31⁄2 lbs.

Chyle. As a result of the process of digestion, the oleaginous matters which have been acted upon by the pancreatic juice and bile are trans

formed into a condition of emulsion, forming an opaque, milky fluid, termed chyle, which adheres to the folds of the mucous membrane and villi.

The Molecules of the fat are first absorbed by the epithelial cells upon the surface of the villi, through which they pass and enter the lymphatics.

Absorption by the Lacteals. The lacteals, or lymphatics of the small intestine, have their origin in the interior of the villi, from which they emerge and form a lymphatic plexus; the larger branches of which pass through the layers of the mesentery, and finally terminate in the thoracic duct (Fig. 6).

In the intervals of digestion the lacteals contain clear, transparent lymph, and are invisible on account of their small size and delicacy. But during digestion these vessels become filled, from absorption of the chyle, and form a visible network of white vessels ramifying through the mesentery, and converging toward the receptaculum chyli.

The lacteal vessels also absorb a small quantity of water, albuminose, glucose and salts.

The Products of digestion find their way into the general circulation by two routes:

1. Water, albuminose, glucose and salts are mainly absorbed by the gastric and mesenteric veins, carried into the liver, through the capillaries of which they pass, and enter the inferior vena cava by the hepatic veins.

2. The Fats are absorbed by the lacteals, emptied into the thoracic duct, and enter the blood at the junction of the internal jugular and subclavian veins.

Forces aiding the movements of Lymph and Chyle.

1. Endosmosis. The continued transudation of matter from the capil. laries, and its absorption into the lymphatics by endosmosis, constitutes the main cause, the vis-a-tergo, of the movement of the lymph; it is so considerable as to rupture the walls of the vessels if they are ligated.

2. Contraction of the non-striated muscular fibres in the walls of the lymphatic vessels, especially when fully distended, aided by the action of the valves, promotes the onward flow of the fluids.

3. Muscular contraction in all parts of the body, by exerting intermittent pressure upon the lymphatic vessels, hastens the current onward; regurgitation being prevented by the closure of the valves.

4. The inspiratory movement, by expanding the chest, causes a dilation of the thoracic duct, and a rapid flow of lymph and chyle into it; during expiration it is compressed, and the fluids forcibly expelled into the venous system.

BLOOD.

The Blood is a nutritive fluid containing all the elements necessary for the repair of the tissues; it also contains principles of waste absorbed from the tissues, which are conveyed to the various excretory organs and by them eliminated from the body.

The total amount of blood in the body is estimated to be about oneeighth of the body weight; from 16 to 18 pounds in an individual of average physical development. The quantity varies during the 24 hours; the maximum being reached in the afternoon, the minimum in the early morning hours.

Blood is a heterogeneous, opaque red fluid, having an alkaline reaction, a saline taste, and a specific gravity of 1.055.

The opacity is due to the refraction of the rays of light by the elements of which the blood is composed. The color varies in hue, from a bright scarlet in the arteries to a deep purple in the veins, due to the presence of a coloring matter, hæmoglobin, in different degrees of oxidation.

The alkalinity is constant, and depends upon the presence of the alkaline sodium phosphate, Na, HP 04.

The saline taste is due to the amount of sodium chloride present.

The specific gravity ranges within the limits of health, from 1.045 to 1.075.

The odor of the blood is characteristic, and varies with the animal from which it is drawn, due to the presence of caproic acid.

The temperature of the blood ranges from 98° Fahr. at the surface to 107° Fahr. in the hepatic vein; it loses heat by radiation and evaporation as it approaches the extremities, and as it passes through the lungs.

Blood consists of two portions :—

1. The Liquor Sanguinis or Plasma, a transparent, colorless fluid, in which are floating

2. Red and white corpuscles; these constituting by weight less than one-half, 40 per cent., of the entire amount of blood.

Water acts as a solvent for the inorganic matters and holds in suspension the corpuscular elements.

Albumen is the nutritious principle of the blood; it is absorbed by the tissues to repair their waste, and is transformed into the organic basis characteristic of each structure.

Paraglobulin or fibrinoplastin is a soft amorphous substance precipitated by sodium chloride in excess, or by passing a stream of carbonic acid through dilute serum.

Fibrinogen can also be obtained by strongly diluting the serum and passing carbonic acid through it for a long time, when it is precipitated as a viscous deposit.

Fatty matters exist in small proportion, except in pathological conditions and after the ingestion of food rich in oleaginous matters; it soon disappears, undergoing oxidation, generating heat and force, or is deposited as adipose tissue.

Sugar is represented by glucose, a product of the digestion of saccharine matter and starches in the alimentary canal; glycogenic matter is derived from the liver.

The Saline constituents aid the process of osmosis, give alkalinity to the blood, promote the absorption of carbonic acid from the tissues into the blood, and hold other substances in solution; the most important are the sodium and potassium chlorides, the calcium and magnesium phosphates.

Excrementitious matters are represented by carbonic acid, urea, creatin, creatinin, urates, oxalates, etc.; they are absorbed from the tissues by the blood and conveyed to the excretory organs, lungs, kidneys, etc.