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A Letter from the late Mr. William Hewsoh, F.R.S., and Teacher 

 of Anatomy in London, to Dr. John Haygarth, Physician in 

 Chester (CL). 



My dear Sir, I have now a little leisure, and shall endeavour 

 to fulfil my promise, by sending you a sketch of my observations. 



The red particles of the blood, improperly called globules, 

 are flat in all animals, and of very different sizes in different 

 animals. In man, they are small, as flat as a shilling, and 

 appear to have a dark spot in the middle. In order to see 

 them distinctly, I dilute the human blood with fresh serum. 

 My predecessors, not having thought of this, could not see 

 them distinctly, and Leeuwenhoek in particular, imagining a 

 round figure fittest for motion, concluded they must be round 

 in the human body ; though he and others allowed, that, in 

 frogs, &c. where they viewed them distinctly, from the blood 

 being thinner, they were flat. Now, I prove that they are flat 

 in all animals. In the human blood, where these particles are 

 small, it is difficult to determine what that black spot is, which 

 appears in the centre of each. Some have concluded that it 

 was a perforation ; but in a frog, where it is six times as large 

 as in a man, it is easy to show that it is not a perforation, but, 

 on the contrary, is a little solid, which is contained in the 

 middle of a flat vesicle. Instead, therefore, of calling this part 

 of the blood red globules, 1 should call it red vesicles ; for each 

 particle is a flat vesicle, with a little solid sphere in its centre. 



(CL.) From the * Medical and Philosophical -Commentaries, by a 

 Society in Edinburgh/ vol. iii, p. 87, 8vo, Bondon, 1775. In the 

 General Index to the present edition of Mr. Hewson's works, references 

 will be found to the pages in the Third Part of the * Experimental 

 Inquiries,' where the different points mentioned in this Letter are more 

 fully discussed. 



