160 PHYSIOLOGY. 



CLII. In the course of the venous blood there is also some 

 variety. 1. In the small veins, the blood is liable to have a 

 motion retrograde to its usual direction (CXLVIII.), as in 

 the arteries (CLI. 2.). 2. The blood returning to the heart 

 from most parts of the body, passes successively from smaller 

 into larger veins, by a series of pretty regularly increasing ves- 

 sels, till they form the vena cava entering the heart. But 

 this is varied in the abdomen, where the veins carrying the 

 blood returning from every viscus included in that cavity, ex- 

 cept the kidneys and genitals, unite in forming the vena por- 

 tarum, whereby they undergo a peculiar distribution. 3. The 

 veins returning the blood from the extreme arteries in the brain 

 do not carry it to the heart by a series of regularly increasing 

 vessels, but by the interposition of sinuses, into which the 

 small veins issuing from the cortical part of the brain imme- 

 diately pour their blood. 4. As the course of the blood 

 through the vessels of the lungs is not at all times equally free, 

 and, particularly, is considerably interrupted at the end of ex- 

 piration, so at the same time it is also interrupted at entering 

 the right ventricle of the heart ; and this often occasions some 

 regurgitation, or retrograde motion, in both the ascending and 

 descending cava. 



CLIII. The whole of the fluids carried in the aorta to its ex- 

 treme branches, are not returned again by the continuous veins 

 to the heart, as, by secretory vessels, a part of them is con- 

 stantly carried out of the course above described. Some of 

 these secreted fluids are thrown entirely out of the body, and 

 others are poured into certain cavities, for various purposes of 

 the economy ; and some of these are again returned into the 

 course of the circulation. Of these last, there is a peculiar 

 fluid which, from the extremities of the arteries, is poured out 

 in a liquid form, or exhaled in that of vapour, into, perhaps, 

 every cavity and vacuity of the body. This, after having ser- 

 ved the purpose of the effusion, seems intended to be regularly 

 returned again into the course of the circulation ; and, accord- 

 ingly, in all the several cavities into which it had been effus- 

 ed, there are absorbents which again take it in. These do not 

 carry the fluid immediately into the veins, but uniting form the 

 vessels called LYMPHATICS, which, in their course, pass through 



