26 ZOOLOGY. 



reproductive organs, and the renals from the kidneys : both 

 of these are paired (i.e. both right and left), but owing to 

 the asymmetrical arrangement of the kidneys, the renal 

 veins are also imperfectly symmetrical. Dor so-lumbar 

 veins from the body-wall join the renals. Then come in 

 the hepatic veins from the liver ( 7), and the phrenic from 

 the diaphragm. Besides all these, a number of median dorsal 

 veins enter it at intervals corresponding to the vertebrae. 



9. The Lymphatic System. We have now traced, in 

 a general way, the veins from all parts of the body, except 

 the lungs, to the heart. But all the blood-plasma carried 

 outwards by the arteries does not travel this way back, 

 although the corpuscles do. We have seen that some of 

 the plasma exudes through the capillary-walls and bathes 

 the tissues ; part of this is assimilated by the tissues, but 

 the rest, carrying with it the products of tissue -waste 

 (katastases), travels back to the blood by a separate series 

 of vessels, the lymphatics^ These start as capillaries, quite 

 unconnected with the blood-capillaries, and mostly blind, 

 though some lead out from the coelom by " lyniph-stomata " 

 (see fig. 11, p. 48). They unite into larger and larger 

 vessels, which finally open into the precaval veins at their 

 origin. Along the course of the lymphatics there occur at 

 intervals whitish lumps called " lymph-glands " (though 

 strictly they are not true glands), and in these white 

 corpuscles are produced and added to the stream of lymjyJ^ 

 as the fluid in these vessels is called. The lymph islorced 

 along, partly by the general pressure of the tissues on them, 

 and partly by contraction of the walls of the lymphatics. 

 After death, as the blood accumulates in the veins, it may 

 often force its way into the main lymphatics, but only 

 under such abnormal conditions (often seen in dissecting) 

 do the lymphatics contain red blood. The main lymphatic 

 of the neck, with the " glands " on its course, can easily be 

 traced in dissecting, but owing to the absence of colour the 

 lymphatics are mostly difficult to trace. 



There are three main lymphatic trunks into which the 

 lymph from all parts eventually flows. Two of these run 

 on the right and left sides of the neck. The third runs in 



