by Bernard' in the submaxillary gland of the dog. During the ordinary condition of glandular repose he found that it required sixty-five seconds to obtain five cubic centimetres of blood from the submaxillary vein; but, when the gland was excited to functional activity, the same quantity of blood was discharged by the vein in fifteen seconds. Thus the volume of blood passing through the organ in a given time was more than four times as great while the gland was in a state of active secretion, as in a condition of repose.

The increased flow of blood, in a secreting gland, is accompanied also by an important change in its appearance. During repose, the blood, which enters the submaxillary gland from the arteries bright red, is changed in its tissue from arterial to venous, and passes out by the veins of a dark color. But when the secretion of the gland is excited, either by galvanization of its nerve or by introducing vinegar into the mouth of the animal, the blood is not only discharged in larger quantity, but passes out red by the veins, so as hardly to be distinguished in color from arterial blood. When the secretion of the gland is suspended, the blood in its vein again becomes dark-colored as before. There is little doubt that the same is true of most of the secreting glands, and that the blood circulating in their capillaries is changed from red to blue only during the period of functional repose; while at the time of active secretion it not only passes through the vessels in greater abundance, but also retains its ruddy color in the veins.

This is because, during the period of glandular repose, the blood performs in its tissues the usual functions of nutrition. It therefore undergoes the ordinary changes and becomes altered in color from arterial to venous. But the period of active secretion is a period of congestion, during which the blood passes in larger quantity, while its watery and saline ingredients exude into the secretory ducts, bringing with them the materials accumulated in the interval of repose. There is nothing in this process to exhaust the oxygen of the blood or to change its color from arterial to venous, and it therefore passes into the veins comparatively unaltered.

A similar ruddy color of venous blood is to be seen in the renal veins, where it is often nearly identical with that of arterial blood. The dif ference in hue between the renal veins and the neighboring muscular veins or the vena cava, when exposed by opening the abdomen of the living animal, is very marked, provided the kidneys be at the time in a state of functional activity. The greater part of the blood traversing these organs is changed only by the elimination of its urea and the remaining ingredients of the urine, which exude into the excretory tubules. The process of active local nutrition is here altogether subservient to the discharge of organic materials already existing in the blood; and the loss of oxygen and alteration in color of the circulating fluid are thus comparatively insignificant.

1 Leçons sur les Liquides de l'Organisme. Paris, 1859, tome ii. p. 272.

Fig. 125.

On the other hand, the venous blood coming from the muscular tissue is very dark colored, especially if the muscles be in a state of active contraction. As the muscles form so large a part of the entire mass of the body, their condition has a preponderating influence upon the color of the venous blood in general. The greater the activity of the muscular system, the darker is the color of the blood returning from the trunk and extremities. When the muscles are in a state of repose or paralysis, on the contrary, the change is less marked; and in the complete relaxation produced by abundant hemorrhage or by complete etherization, the blood in the veins often approximates in color to that in the arteries.

Finally, in the lungs the reverse process takes place. In these organs the blood is supplied with a fresh quantity of oxygen, to replace that which has been consumed elsewhere; and accordingly it changes its color from dark purple to bright red as it passes through the pulmonary capillaries.

Both the physical and chemical phenomena, therefore, of the circulation vary at different times and in different organs. The actions which go on throughout the body, are varied in character, and produce a similar difference in the phenomena of the circulation. The venous blood, consequently, has a different composition as it returns from different organs. In the parotid gland it yields the ingredients of the saliva; in the kidneys those of the urine. In the intestine it absorbs the nutritious elements of the digested food; and in the liver it gives up substances destined to produce the bile, while it absorbs. glucose from the hepatic tissue. In the lungs it changes from blue to red, and in the capillaries of the general system, from red to blue; and its temperature, also, varies in different veins, according to the peculiar chemical and nutritive changes going on in the organs from which they originate.

DIAGRAM OF THE CIRCULATION.-1. Heart. 2. Lungs. 3. Head and upper extremities. 4. Spleen. 5. Intestine. 6. Kidney. 7. Lower extremities. 8. Liver.

CHAPTER XVI.

THE LYMPHATIC SYSTEM.

In addition to the connected series of canals by which the blood passes in a continuous round through the arteries, capillaries, and veins, there is also a system of vessels, leading only from the periphery toward the centre, and discharging into the great veins near the heart the fluids which have been absorbed in the solid tissues of the body. The fluid contained in these vessels is nearly or quite colorless, especially in thin layers, and from its transparent and watery appearance is called the “lymph," and the vessels themselves constitute what is known as the lymphatic system.

As the blood circulates through the capillaries under the influence of the arterial pressure, certain of its ingredients transude through the vascular walls and penetrate the interstices of the anatomical elements of the tissues. An increased pressure upon the blood, either from arterial congestion or from obstruction to the venous current, will increase the amount of transudation, producing an œdematous condition of the part, which is first perceptible in the loose connective tissue, but which may afterward involve the more compact substance of the organs. In the normal state of the circulation, this interstitial fluid, which is the real source of nutrition for the solid parts, does not, however, stagnate in contact with them, but is renewed by a continual change. As fresh supplies need to be drawn from the circulating blood, the older portions are removed by absorption and returned to the centre of the circulation by the lymphatic vessels. Thus these vessels may be considered as complementary in their function to the veins. The blood, containing the red globules, requires to be rapidly and abundantly returned to the lungs by the veins, in order to regain the oxygen necessary for its continued vitality; while the lymphatics collect more gradually the fluids which have served for the slower process of nutrition and growth.

Anatomical Structure and Arrangement of the Lymphatic System. In structure the lymphatics do not essentially differ from the bloodvessels, their principal peculiarity being that their walls are more delicate and transparent. This circumstance, together with the colorless nature of their contents, renders them less easily recognizable by dissection. Those of larger and medium size consist of three coats, similar, in general characters, to the corresponding tunics of the blood vessels. According to the observations of Kölliker, the external coat alone is distinguished from that of the veins by the possession of smooth mus( 354 )

cular fibres which are arranged in a longitudinal and oblique direction; a character which is to be seen in lymphatics of 0.2 millimetre in diame ter and upward. Like the veins, they are provided with numerous valves, opening toward the heart and closing toward the periphery, the vessel often presenting a well-marked dilatation just within the situation of the valves. The smallest lymphatics consist of only a single coat, composed of flattened, epithelium-like, nucleated cells, which may be brought into view, like those of the capillary blood vessels, by the staining action of a silver nitrate solution.

Origin and Course of the Lymphatic Vessels.-So far as the origin of the lymphatics has been demonstrated by injections, these vessels commence in the substance of the tissues by plexuses. They are more abundant in organs which are fully supplied with blood vessels, and are absent in tissues where blood vessels do not exist, such as those of the cornea, the vitreous body, and the epithelial coverings of the skin and mucous membranes. According to Von Recklinghausen, the meshes of the lymphatic plexus, as a general rule, are intercalated between those of the capillary blood vessels; so that the point of junction of two or more lymphatics is always in the middle of the space surrounded by the adjacent blood vessels. Thus the lymphatic capillary is situated at the greatest distance possible from the nearest capillary blood vessels; and in the transudation of fluids from one to the other, the intervening substance of the tissue will always be completely traversed by the nutritious ingredients of the blood. In membranous expansions presenting a free surface, as in the skin and mucous membranes, the plexus of capillary blood vessels is invariably nearer the surface, while the lymphatics occupy a deeper plane beneath it. Even in the villi of the small intestine, the network of blood vessels is situated immediately under the epithelial layer, and surrounds the lacteal vessel which is placed in the central part of the villus.

Beside the lymphatic capillaries proper, certain irregularly shaped spaces or canals, containing only a colorless or serous fluid, have been found in organs consisting of condensed connective tissue, like the central tendon of the diaphragm and muscular fasciæ. They have been demonstrated mainly by the process of treating the tissues with a solution of silver nitrate, which stains the solid portions of a dark color, but leaves the capillary vessels and the serous canals uncolored. These interstitial spaces or serous canaliculi have been regarded by some observers (Recklinghausen) as directly continuous with the lymphatic capillaries, and as constituting the immediate sources of supply for the lymph; but this connection is not universally admitted. The serous canaliculi are distinguished from the lymphatic capillaries by their much smaller size, and by the fact that they do not possess, like the latter, a lining of epithelial cells.

From their plexuses of origin the lymphatic vessels pass inward toward the great channels and cavities of the body, uniting into larger branches and trunks, and following generally the course of the prin

cipal bloodvessels and nerves. Those of the lower extremities enter the cavity of the abdomen, and join with the lymphatics of the abdominal organs to form the commencement of the thoracic duct, which ascends through the cavity of the chest, receiving branches from the thoracic organs to the root of the neck, where it is joined by lymphatics from the left side of the head and the left upper extremity, and terminates in the left subclavian vein, at the point of its junction with the left internal jugular. The lymphatics coming from the right side of the head and neck, the right upper extremity, and a portion of the thoracic organs, form a trunk of smaller size, the right lymphatic duct, which terminates in the right subclavian vein at its junction with the right internal jugular. Thus the lymph, collected from the vascular tissues of the entire body, is mingled with the venous blood a short distance before its arrival at the right side of the heart.

The Great Serous Cavities of the Body are Lymphatic Lacunæ.— It is well known that in the amphibious reptiles there are irregularlyshaped spaces or lacunæ, forming a part of the lymphatic system and interposed between adjacent organs in various parts of the body. In the mammalia the peritoneal and pleural cavities, and probably all the principal serous sacs, are also in direct communication with the lymphatic vessels. This was first shown by Recklinghausen' for the peritoneal cavity of the rabbit, which communicates by microscopic orifices with the lymphatic vessels in the central tendon of the diaphragm. These communications were demonstrated in two ways: First, on injecting into the peritoneal cavity of the animal milk, or a watery fluid holding in suspension minute granules of coloring matter, the lymphatic vessels of the central tendon of the diaphragm were afterward found to be filled with the white or colored injection. Secondly, the central tendon of the diaphragm being carefully removed from the recently killed animal, and a drop of milk placed upon its peritoneal surface, the milk globules could be directly observed under the microscope, running in converging currents to certain points on the surface of the tendon and there penetrating into its lymphatic vessels. The cavity of the pleura has also been found by similar means to communicate with the lymphatic vessels in its neighborhood. The serous cavities accordingly are either extensive lacunæ, forming in some regions the origin of the lymphatic vessels, or else they are wide but shallow expansions of the cavity of the lymphatics, situated at various points in their course.

The Lymphatic Glands.-During the passage of the lymphatic vessels from the periphery toward the centre, they are repeatedly interrupted by ovoid, glandular-like bodies, of a pale reddish color and somewhat firm consistency, varying in size from about two to twenty millimetres in their long diameter. They do not exist in fish and reptiles, but are always present in birds and mammalia. As a rule, several lymphatic vessels reach these bodies, coming in a direction from the periphery; and

1 Stricker's Manual of Histology, Buck's Edition. New York, 1872, p. 221.