S. Hales, W. Hewson, T. Young 299 



others, and Professor Langley has summarized the work 

 of some of them in his " Sketch of the progress of the 

 discovery in the eighteenth century of the autonomic 

 nervous system." 1 



In the eighteenth century a most distinct advance in 

 animal physiology was made north of the Tweed by Joseph 

 Black, whose work in Physics and Chemistry has already 

 been described (see p. 65). Investigating the properties of 

 carbonic acid gas or " fixed air," as it was then called, he 

 noted that " fixed air " is also present in expired air, and 

 is physiologically irrespirable, though not toxic. 



William Hewson (1739-1774), a pupil of the Hunters (see 

 Chapter X.), became assistant to them, and John Hunter 

 left him in charge of his dissecting room when abroad with 

 the army. For a time Hewson was in partnership with 

 William Hunter. It was he who discovered the existence of 

 lymphatic and lacteal vessels in birds, reptiles and fishes, 

 a fact which was of great importance in view of the opinions 

 held by the Hunters that absorption is the function of these 

 vessels ; for hitherto the opponents of this view had pointed 

 to the absence of these organs in the lower vertebrates. 

 A more important work was embodied in his experimental 

 enquiry into the properties of blood (1771). Hewson showed 

 that when coagulation of the blood is delayed by cold or by 

 the addition of neutral salts, a coagulable fluid may be 

 separated from the corpuscles. He further showed this 

 fluid was an insoluble substance which could be precipitated. 

 According to Hewson's view, coagulation was due to the 

 formation of this substance which he called " coagulable 

 lymph," and which we now call " fibrinogen." For a time 

 his work was forgotten, but now at last its value is fully 

 recognized. 



The Quaker physician, Thomas Young, whose brilliant 

 work in Physics has been described in our first chapter, was 

 the founder of the science of Physiological Optics. He 

 studied under John Hunter, and amongst his early discoveries 

 he showed that the accommodation of the eye to different 

 distances is due to changes in the curvature of the crystalline 



1 Journal of Physiology, Vol. L., 1916. 



