VOL. L.] PHILOSOPHICAL TRANSACTIONS. 147 



serum ; and this subordination is, according to the same laws, continued down 

 through fluids more subtle than the lymph, to the smallest vessel, which is pro- 

 pagated from the aorta. Such was Boerhaave's doctrine concerning the vascular 

 system of minimal bodies; like many of his other notions, ingenious, plausible, 

 and recommending itself, at first sight, by an appearance of geometrical and 

 mechanical accuracy, but founded on insuflicient data, and by no means to be 

 reconciled to appearances. 



For, in the first place, should we admit his hypothesis, it is certain, that the 

 conical or converging form of the aorta, and the change of direction in its 

 branches, must, in the distant blood-vessels, occasion a great resistance to the 

 moving blood, and a great diminution of its velocity. Suppose that this resist- 

 ance be, in any capillary red artery, to the resistance in the trunk of the aorta, 

 as any larger assignable number is to unit : the resistance, then, in a capillary 

 serous artery will, to that in the aorta, be as the square of that number is to 

 unit ; in the capillary lymphatic, as the cube ; and so in progression : that is, 

 the velocity of the fluids, in the remoter series of vessels, will be, physically, 

 nothing. But we know, on the contrary, that some very remote series of ves- 

 sels have their contents moved with a very considerable velocity ; particularly the 

 vessels of the insensible perspiration: and in anatomical injectictns, the liquor 

 thrown into an artery scarcely returns more easily or speedily by the correspond- 

 ing vein than by the most subtle excretory ducts. Moreover, there are an infi- 

 nite number of observations of morbid cases, in which the red blood itself has 

 been evacuated through some of the most remote series of vessels, merely from 

 an occasional temporary obstruction in one part, or a preternatural laxity in an*- 

 other; and without any lasting detriment to the structure and subordination of 

 the vessels; which yet, upon this hypothesis, must have been utterly destroyed 

 before such an irregularity could have happened. 



The other theory concerning the origin of the lymphatics has been main- 

 tained by some very eminent physiologists later than Boerhaave ; and supposes, 

 that these vessels receive their lymph from the blood-vessels, or from the excre- 

 tories of the larger glands, by the intermediation of only one small vessel, which 

 these authors term a lymphatic artery, invisible in its natural state, nor yet ren- 

 dered subject to the senses by experiments. But to this it may be answered, 

 that the lymphatics are traced into many parts of the body, and lost there; and 

 therefore most probably have their origin there, where no large gland nor blood- 

 vessel is to be found in their neighbourhood: that it contradicts the whole ana- 

 logy of nature, to suppose the motion of an animal fluid more discernible in the 

 veins than in the arteries; and, finally, that it seems rather an instance of want 

 of thought, and of being imposed upon by words, to call the. lymphatic vessels 

 veins, because they are furnished with valves; and tlien, because they are 

 called veins, to take for granted, that of course they must be the continuation 

 of arteries. 



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