Hewson gave the first complete account of the anatomical peculiarities of the lymphatics. He divided the lymphatics into two groups – superficial and deep. He described the leucocytes as derived from the lymphatic glands and thymus.

"Hewson's studies of the lymphatics are models of skill and ingenuity. He studied them in vitro and in vivo (observing them through a hand‐held magnifying glass in the web of a frog's foot). Disproving the current theory, he showed that the lymphatics are not part of the blood system, that nodes are stopping stations along lymphatic vessels and that every cavity of the body, and not just the lacteals of the small intestine, is drained by the lymphatic system. He demonstrated the absorptive properties of the lymphatics by injecting a dye or noxious substance into experimental animals and then demonstrating it in the lymphatics in a distant part of the body. It was this theory – that the lymphatics, and not just the intestinal lacteals, are a vast and highly effective absorption system – that brought him into conflict with Monro secundus (Wintrobe, 1980).

"Hewson went further, suggesting that noxious agents could enter the body via the lymphatics. ‘The axillary glands are likewise frequently observed to swell in consequence of cancers in the breast and it is found of no use to extirpate the breast itself unless the affected glands can likewise be removed; for otherwise the cancerous tumour of the glands may renew the disease.’ (Hewson, 1774a).

"This work inevitably led him to study further the villi of the gut and their lacteals. He demonstrated how villi in the small intestine differed from those in the colon, and then went on to compare the lacteals of fish, amphibians and turtles (Hewson, 1774b)." (Derek Doyle, "William Hewson (1739-74): the father of haematology", British Journal of Haematology, April, 2006).