The pumping of the blood through the arteries is assisted by the contractions of the muscular coat, while the elastic tissue, of which it contains a certain amount, gives elasticity to the walls and enables them to stretch and so to accommodate the larger blood supply forced into them at each beat by the heart. The walls of the veins have not the power of contracting and the blood is pushed through more by gravity and the action of the arteries than by any action of their own.
The walls of all the vessels are nourished by tiny blood-vessels in the outer coat, known as vasa vasorum, and the nerves that regulate the action of the arteries are the vasomotor nerves from the vasomotor center in the medulla. Sufficient impulse goes from this center to the blood-vessels all the time to keep them somewhat contracted, in a state of tone, that is, which is increased or diminished as the blood supply is to be diminished or increased.
Lymphatic System.—The [lymphatic system] also extends throughout the body and consists of a system of channels, spaces, and glands very closely related to the circulatory system and containing a fluid called lymph. There are three principal parts to the system: 1. the lymph spaces, which are open spaces, with no definite walls, in the connective tissue framework of the body, more frequent near arteries and veins and especially so among the capillaries; 2. the lymph capillaries or small vessels which connect the lymph spaces; and 3. the lymphatic vessels, of which there is a deep and a superficial set, the latter accompanying the superficial veins on the surface of the body, the former accompanying the deep blood-vessels.
Fig. 7.—Diagram showing the course of the main trunks of the absorbent system: the lymphatics of lower extremities (D) meet the lacteals of the intestines (LAC) at the receptaculum chyli (R.C.), where the thoracic duct begins. The superficial vessels are shown in the diagram on the right arm and leg (S), and the deeper ones on the left arm (D). The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck (T). (Yeo.)
The lymph spaces are generally small, though there are some large serous cavities, such as the abdomen, that may be considered as extended lymph spaces.
Fig. 8.—Diagram of a lymphatic gland, showing afferent (a. l.) and efferent (e. l.) lymphatic vessels; cortical substance (C); medullary substance (M); fibrous coat (c); sending trabeculæ (tr) into the substance of the gland, where they branch, and in the medullary part form a reticulum; the trabeculæ are surrounded by the lymph path or sinus (l. s.), which separates them from the adenoid tissue (l. h.). (Sharpey.)
The lymphatic vessels have delicate, transparent walls, with three coats like the arteries, though much thinner, and anastomose even more freely than the veins. They have a beaded appearance due to the presence of numerous valves, which form constrictions on their surface. The right lymphatic duct, which is only about an inch long, drains all the lymphatics of the right half of the upper part of the trunk, the head, and the neck approximately, while the thoracic duct drains those of the rest of the body. The latter, which is the largest vessel of the system, begins opposite the second lumbar vertebra with a bulb-like reservoir for the lymph or chyle, the receptaculum chyli, and extends up along the spinal column for a distance of about eighteen inches to the seventh cervical vertebra, where, with the right lymphatic duct, it empties into the left subclavian vein at its junction with the internal jugular, thus establishing direct communication between the lymph spaces and the venous system. The orifices of both vessels are guarded by semilunar valves to prevent regurgitation of the blood.
