The White Veins: Conceptual Difficulties in the History of the Lymphatics

RECOGNITION of the true nature of the lymphatics depended upon the realization that, despite inter-relations, these vessels were different from veins.This realization evolved slowly.It commenced with sporadic observations by the ancients of the existence of vessels of similar dimensions to small veins but of different appearance.These early references to lymphatics were neglected for about 2,000 years.With the Renaissance, interest in the lymphatics was revived, spurred initially by accidental discoveries of lymph vessels loaded with chyle.Deliberate studies soon followed, making possible a coherent account of the ramifications of the system.Techniques which had been successfully applied to related disciplines were now turned to the study of the new vessels; notable in this connection were microscopy and intravascular injections.The latter, albeit much modified, have been employed in researches on the lymphatics ever since.The ability to distinguish lymphatics and veins on an anatomical and distributive basis was soon established.However, while their function remained obscure it was possible to regard lymphatics as merely another sort of vein.This conception was widely held despite serious objections until well into the eighteenth century.The Hunterian School of Anatomy finally dissipated this notion with the demonstrations of the absorbing capacity of the lymphatics.From then onwards the differences between lymphatics and veins were widely realized.There remained only the problems of clarifying the precise relations of the two systems, a task still incomplete as modern investigators dispute the existence and raison d'etre of lympho-venous communications.EARLIEST REFERENCES Lymphatics were observed long after the rest of the vasculature was known to exist.Their smallness and transparency were principally responsible for this delay, although early references to their existence were neglected as the significance of these tiny vessels was not realized.Aristotle (384-322 B.C.) iS said to have described structures containing colourless fluid which could have been lymphatics.Likewise, there are passages in the books ascribed to Hippocrates which may refer to lacteals, the following being the most suggestive: 'There are also in the body, veins from the stomach, very small, and of all kinds, by whose means the food comes into the body'.However, it is difficult to know which vessels Hippocrates was describing in this quotation as Cruikshank commented in 1786: 'Hippocrates . . .could not mean the real absorbents: no vessels different from arteries and veins are easily to be seen