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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera
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The Lymphatic System


This system of vessels consists, chiefly, of one or two principal trunks, the thoracic duct, or ducts, which underline the vertebral column, and communicate, anteriorly, with the superior venae cavae, or with the veins which open into them.

From these trunks, branches are given off, which ramify through all parts of the body, except the bulb of the eye, the cartilages, and the bones. In the higher Vertebrata, the larger branches are like small veins, provided with definite coats, and with valves opening toward the larger trunks, while their terminal ramifications form a capillary net-work; but, in the lower Vertebrates, the lymphatic channels assume the form of large and irregular sinuses, which not unfrequently completely surround the great vessels of the blood - system.

The lymphatics open into other parts of the venous system besides the affluents of the superior cavae. In Fishes there are, usually, two caudal lymphatic sinuses which open into the commencement of the caudal vein. In the Frog, four such sinuses communicate with the veins, two in the coccygeal, and two in the scapular, region. The walls of these si nuses are muscular, and contract rhythmically, so that they receive the name of Lymphatic hearts. The posterior pair of these hearts, or non - pulsating sinuses corresponding with them, are met with in Reptilia and Aves.

Accumulations of indifferent tissue in the walls of some of the lymphatic sinuses are to be met with in Fishes; but it is only in the Crocodilia, among Reptilia, that an accumulation of such tissue, traversed by lymphatic canals and blood-vessels, is apparent, as a Lymphatic gland, in the mesentery. Birds possess a few glands in the cervical region; and, in Mammalia, they are found, not only in the mesentery, but in many parts of the body.

The Spleen is substantially a lymphatic gland. The Thymus - a glandular mass with an internal cavity, but devoid of any duct - which is found in all Vertebrata except Amphioxus, appears to belong to the same category. It is developed in the neighborhood of the primitive aortic arches, and is double in most of the lower Vertebrata, but single in Mammalia.

The nature of two other "ductless glands," the Thyroid gland and the Suprarenal capsules, which occur very widely among the Vertebrata, is by no means well understood.

The thyroid gland is a single or multiple organ, formed of closed follicles, and is situated near the root of the aorta, or the great lingual, or cervical, vessels which issue from it.

The suprarenal capsules are follicular organs, often abundantly supplied with nerves, which appear to occur in Fishes, and are very constant in the higher Vertebrata, at the anterior ends of the true kidneys.

The Lymph Corpuscles, which float in the plasma of the lymphatic fluid, always resemble the colorless corpuscles of the blood.


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