Complex sentence: He was one of the seven physicians who aided Francis Glisson in the observations preparatory to the publication of the "Tractatus de Rachitide" (on rickets) in 1650; and he was a friend of John Milton, whose third wife was his cousin.
Split into simpler sentences: He was one of the seven physicians who aided Francis Glisson in the observations preparatory to the publication of the "Tractatus de Rachitide" (on rickets) in 1650. Paget was a friend of John Milton, whose third wife was his cousin.