LYMPHATIC SYSTEM. 183 



tion of new vessels in its interstices, and this is effected by similarly metamor- 

 phosed cells, arising in the areolaj and joining- at various points with the 

 surrounding' vessels, and also simply by pointed offshoots from the existing 

 capillaries stretching across the intervals and meeting from opposite sides, so as 

 when enlarged to form new connecting arches. 



The blood-vessels may be said to increase in size and capacity in proportion to 

 the demands made on their service. Thus, as the uterus enlarges in pregnancy, 

 its vessels become enlarged, and when the main artery of a limb is tied, or 

 otherwise permanently obstructed, collateral branches, originally small and insig- 

 nificant, 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. 



LYMPHATIC SYSTEM. 



Under this head we include not only the vessels specially called 

 lymphatics or absorbents, together with the glands belonging to them, 

 but also 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 lymphatic system has 

 already been given at page 37. 



A system of lymphatic vessels is superadded to the sanguiferous in 

 all classes of vertebrated animals, but such is not the case in the inver- 

 tebrata; 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 nearly all the textures and organs 

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

 discovery may yet possibly disappear. It is, however, with the con- 

 nective tissue of the several textures and organs that the lymphatics 

 are most intimately associated ; indeed, as we shall immediately have 

 occasion to notice, these vessels may be said to take origin in spaces in 

 that tissue. The larger lymphatic trunks usually accompany the 

 deeply-seated blood-vessels ; they convey the lymph from the plexuses 

 or sinuses of origin towards the thoracic duct. The principal lymph- 

 atic vessels of a part exceed the veins in number but fall short of them 

 in size; they also anastomose or intercommunicate much more fre- 

 quently than the veins alongside of which they run. 



It not unfrequently happens that a lymphatic vessel may ensheath an 

 artery or vein either partially or wholly. In the latter case the lymph- 

 atic is termed " perivascular." 



Origin. — Two modes of origin of lymphatic vessels are described, 

 viz., the pJexiform and the lacunar. 



Plexiform. In this mode of origin, which is by far the most general, 

 and is met with in its most typical form on the surface of aponeurotic 

 structures such as the central tendon of the diaphragm (fig. 12-i), the 

 vessels begin in form of irregular networks or plexuses Avhich are at 

 various points in connection with the cell-spaces of the connective 



