Of the Lymphatics. 



THE Lymphatics have derived their name from the appearance of the fluid 

 contained in their interior (lympha, water). They are also called absorbents, from 

 the property they possess of absorbing certain materials for the replenishing of 

 the blood, and conveying them into the circulation. 



The lymphatic system includes not only the lymphatic vessels and the glands 

 through which they pass, but also the lacteal or chyliferous vessels. The lacteals 

 are the lymphatic vessels of the small intestine, and differ in no respect from the 

 lymphatics generally, excepting that they contain a milk-white fluid, the chyle, 

 during the process of digestion, and convey it into the blood through the thoracic 

 duct. 



The lymphatics are exceedingly delicate vessels, the coats of which are so 

 transparent that, the fluid they contain is readily seen through them. They retain 

 a nearly uniform size, being interrupted at intervals by constrictions, which give 

 to them a knotted or beaded appearance. These constrictions correspond to the 

 presence of valves in their interior. Lymphatics are found in nearly every texture 

 and organ of the body, with the exception of the substance of the brain and 

 spinal cord, the eyeball, cartilage, tendon, membranes of the ovum, the placenta, 

 and umbilical cord, the nails, cuticle, and hair. Their existence in the substance 

 of bone is doubtful. 



The lymphatics are arranged into a superficial and deep set. The superficial 

 lymphatics, on the surface of the body, are placed immediately beneath the integu- 

 ment, accompanying the superficial veins ; they join the deep lymphatics in certain 

 situations by perforating the deep fascia. In the interior of the body, they lie in 

 the submucous areolar tissue, throughout the whole length of the gastro-pulmonary 

 and genito-urinary tracts ; or in the subserous areolar tissue, beneath the serous 

 membrane covering the various organs in the cranial, thoracic, and abdominal 

 cavities. These vessels probably arise in the form of a dense plexiform network 

 interspersed among the proper elements and bloodvessels of the several tissues; 

 the vessels composing which, as well as the meshes between them, are much larger 

 than those of the capillary plexus. From these networks small vessels emerge, 

 which pass, either to a neighboring gland, or to join some larger lymphatic trunk. 

 The deep lymphatics, fewer in number, and larger than the superficial, accompany 

 the deep bloodvessels. Their mode of origin is not known; it is, however, 

 probably similar to that of the superficial vessels. The lymphatics of any part 

 or organ exceed, in number, the veins; but in size, they are much smaller. 

 Their anastomoses also, especially of the large trunks, are more frequent, and are 

 effected by vessels equal in diameter to those which they connect, the continuous 

 trunks retaining the same diameter. 



The lymphatic vessels, like arteries and veins, are composed of three coats. 



The internal is an epithelial and elastic coat ; it is thin, transparent, slightly 

 elastic, and ruptures sooner than the other coats. It is composed of a layer of 

 elongated epithelial cells, supported on a simple network of elastic fibres. 



The middle coat is composed of smooth muscular and fine elastic fibres disposed 

 in a transverse direction. 



The external or areolo-fibrous coat consists of filaments of areolar tissue, 

 intermixed with smooth muscular fibres, longitudinally or obliquely disposed. It 

 forms a protective covering to the other coats, and serves to connect the vessel 

 with the neighboring structures. 



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