42 EFFECTS OF RESPIRATION AND EXERCISE ON TIIE BLOOO. 



of some one part or other of the veins into the ailjacent part; but these vessels 

 being furnished with innumerable valves, within an inch, or less, of each 

 other, the doors whereof open only towards the heart, the blood must neces- 

 sarily force itself out that way, and no other. If we suppose that any given 

 portion of the vein is hereby emptied ; what follows from the circumstance, but 

 that the next-door valve, which kept back the blood contained in the adjoining 

 portion below, while it was pressed upon from above, being thus relieved from 

 the pressure, will now open 1 Then, in rushes the blood, and the space is 

 again filled, but only to be emptied in a similar manner. It is with a view to 

 accelerate the circulation through the veins, and to keep the blood warm in 

 winter, that mankind betake themselves to forced exercises, as hunting, run- 

 ning, or beating the arms athwart the chest, and that beasts with the same 

 propensity gambol and frisk about, or rub themselves: both promote the same 

 ends, by brea hing short, so as not to cool the lungs too much, or by drawing 

 in the same warm air over again, in sheltered situations. 



44. By the process just described, the blood being once admitted into the 

 minuter veins, finds its way to the larger ones, which convey it to the heart; 

 near to which, as the quantity becomes great in the large vein, it receives a 

 powerful auxiliary in the filling and emptying of the lungs, and the working 

 forward and backward of the midriff" as described before. Passing through 

 it near the back bone, and, consequently, as its more muscular part, the great 

 vein must at every inspiration receive from the midriff considerable aid in 

 pouring forth its contents — to say nothing additional of the pump-like action 

 of the heart itself At this part the vein acquires the appearance of a double 

 tube, the outside thickened and muscular, the inner one membranous and col- 

 lapsed, as if too big for the space in which it is placed. But the curious fact — 

 how the blood which had been sent into circulation through the arteries, gets 

 into the veins at first, deserves consideration ; as this must be eflfected labori- 

 ously, when the fluidity of the blood is lessened, or else accelerated with fright- 

 ful rapidity, whenever it so happens that fever prevails : local inflammation, 

 by the same rule, must cause an unusual flow of blood to the part affected ; 

 and, as most of the impurities of the system will then be drawn towards it, at 

 that place must they leave the occasion of the most direful effects. In proof 

 of this doctrine, it happens frequently, that when an animal is attacked with 

 inflammatory complaints at two places at once, the greater evil of the two in- 

 creases, while the lesser one " runs ofl^" as it is called, or gets cured of itself. 

 So, on the contrary, when a diseased horse (glanderous, for example) is well 

 fed and well kept, he not unfrequently overcomes his disorder without medi- 

 cine, to the great wonder of the unknowing ; the solution whereof is, that the 

 constant supply of new blood has quite changed the nature of the animal, hi3 

 disease has been " taken up," or absorbed, by the animal system, and ulti- 

 mately carried oflTin the common evacuations. In short, absorption and efl^u- 

 eion are the great internal secrets of animal life; although we can not say with 

 certainty how they are carried on, it is no less a positive law of nature, that 

 the veins at their commencement in the glands absorb or suck up, the blood 

 that is deposited there by the arteries, and separated by the lymphatics; 

 equally well known is it, that efl!'usion takes place, of arterial blood, into much 

 larger veins, as well as from one part of the system into another, internally. 

 These points are deducible from a multitude of facts and well-known opera- 

 tions, some proofs of which the reader will find more in detail in the course 

 of this treatise. In some parts, a positive connexion of the capillaries, or 

 smallest blood-vesssels, is found to exist; in such a manner, that the secf^.on 

 of one or more of these, and the consequent obstruction of the means of 

 life to all the parts below it, which some would naturally expect, is hereby 

 remedied. 



