148 PHILOSOPHICAL TRANSACTIONS. [aNNO 1757. 



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In attempting to investigate matters too subtile for the cognizance of our 

 senses, the only method, in which we can reasonably proceed, is by inferring 

 from what we know in subjects of the same nature: and our conclusion thus in- 

 ferred, concerning the subject sought, will be firmer and more unquestionable, 

 in proportion as it resembles the subject known. But if the subjects be really of 

 the same kind; if no difference can be shown between them, in any respect 

 material to the inquiry, in which we are engaged; in this case our inference from 

 analogy becomes the very next thing to a physical certainty: and this he appre- 

 hends to be true in relation to the problem before us, concerning the origin of 

 the lymphatic vessels. Though in general we cannot, by experiments, arrive at 

 the extremities of those tubes, nor satisfy ourselves, by inspection, in what 

 manner they receive their fluid; yet in a very considerable number of them we 

 can do both. There is a certain part of the human body very abundantly pro- 

 vided with lymphatics; in which part we can actually force injections through 

 those vessels into a cavity, where their extremities open : and from this cavity, 

 on the other hand, we can at pleasure introduce a coloured liquor into their ex- 

 tremities, and trace it from smaller into wider canals ; from capillary tubes, with- 

 out valves, into large lymphatic trunks copiously furnished with them. We 

 know likewisd", that into this cavity are continually exhaling an infinite number 

 of w^atery and mucous vessels, both arterial tubes and excretory ducts ; that these 

 keep it moist with a perpetual vapour, which the extremities of those lymphatics 

 are, in the mean time, perpetually imbibing. Does it not seem strange, while 

 these particulars are known and acknowledged by all the world, that the great 

 authors of anatomy and physiology should never have reasoned from them, but 

 should run into complex and obscure suppositions, in order to explain a process, 

 which they may at any time examine with their own eyes.^ But perhaps this in- 

 advertency may be accounted for, if we recollect, that at the time when these 

 vessels, and the structure of this part, were discovered, the lymph, and every 

 thing belonging to it, was utterly unknown; and that the vessels in question 

 were first seen and considered as performing another and more remarkable office; 

 which circumstance, it should seem, has prevented succeeding authors from being 

 duly attentive to them in the capacity of lymphatics. However this be, it is 

 certain, that the lymphatics of the mesentery, commonly called the lacteals, 

 differ from those of the other parts in no one particular, save that occasionally 

 they carry chyle instead of lymph, or rather carry lymph mixed, at stated times 

 (that is, for 2 or 3 hours after the creature has taken food) with an emulsion of 

 vegetable and animal substances, and coloured white by that mixture. At other 

 times, that is, during l6 or 18 hours out of the 24, they contain nothing but 

 lymph ; and are, in every respect, mere lymphatic vessels, not to be distinguished 

 from those in any other part of the body. Their structure is the same: the 

 membrane of which they are formed, their valves, the lymph which they contain, 

 the glands through which they pass, their direction from smaller tubes to larger, 



