clxxxii LYMPHATIC! SYSTEM. 



augment greatly in size, to afford passage to the increased share of blood which they 

 are required to transmit, and by this admirable adaptation of them to the exigency, 

 the circulation is restored. In such cases, an increase takes place in length, as well 

 as in diameter, and accordingly the vessels very commonly become tortuous. 



ABSORBENT OR LYMPHATIC SYSTEM. 



Under this head we include not only the vessels specially called lymph- 

 atics, together with the glands belonging to them, but al.-'O those named 

 lacteal or chyliferous, which form part of the same system, and differ in no 

 respect from the former, save that they not only carry lymph like the rest, 

 but are also employed to take up the chyle from the intestines during the 

 process of digestion and convey it into the blood. An introductory outline 

 of the absorbent system has already been given at page xlvii. 



A system of lymphatic vessels is superadded to the sanguiferous in all 

 classes of vertebrated animals, but such is not the case in the invertebrata ; 

 in many of these, the sanguiferous vessels convey a colourless or nearly 

 colourless blood, but no additional class of vessels is provided for conveying 

 lymph or chyle, at least none such has hitherto been detected. 



Distribution. In man and those animals in which they are present, the 

 lymphatic vessels are found in neaily all the textures and organs which 

 receive blood ; the exceptions are few, and with the progress of discovery 

 may yet possibly disappear. 



In the different regions of the body, and in the several internal viscera, 

 the lymphatics are arranged in a superficial and a deep set. The former 

 run underneath the skin or under the membranous coats immediately enve- 

 loping the organs in which they are found ; the latter usually accompany 

 the deep-seated blood-vessels. The principal lymphatic vessels of a part 

 exceed the veins in number, but fall short of them in size ; they also anasto- 

 mose or intercommunicate much more frequently than the veins alongside 

 of which they run. 



Origin. Lymphatics may arise superficially, i. e. immediately underneath 

 free surfaces, both external and internal, as for example those of the skin 

 and mucous membranes, or deeply, in the substance of organs. 



Plexiform origin. When they arise superficially, the lymphatics most 

 generally begin in form of networks or plexuses, out of which single vessels 

 emerge at various points and proceed to enter lymphatic glands or to join 

 larger lymphatic trunks. Such mode of commencement may be termed the 

 plexiform. The plexuses for the most part consist of several strata, becoming 

 finer as they approach the surface, in respect both of the calibre of the 

 vessels and the closeness of their reticulation. This is shown in figure c., 

 which is meant to represent the lymphatic plexuses of the skin. But evtn 

 the most superficial and finest network is composed of vessels which are 

 larger than the sanguiferous capillaries. 



The short anastomosing branches of these plexuses are often of very un- 

 equal size, even in the same stratum, some being dilated and almost saccular, 

 whilst others immediately communicating with these are narrow, so that the 

 network may assume a varicose character. In some situations the plexuses 

 have much the appearance of strata of intercommunicating cellular cavities, 

 and a characteristic example of this appearance is afforded by the intestine 

 of the turtle after its lymphatics have been injected with mercury ; these 

 vessels are then seen to emerge from what has all the appearance of a dense 

 stratum of small rounded cells filled with mercury and lying beneath the 

 surface of the mucous coat. This appearance, however, may be regarded as 



