244 THE ENGINES OF THE HUMAN BODY 



explanation of the heart sky-high and replaced it by the 

 modern conception that the heart is a living mechanical 

 pump. The heart is a simple and familiar kind of 

 machine compared to the nervous system. Yet it was not 

 until the close of the nineteenth century, when inventors 

 succeeded in linking together the inhabitants of great 

 cities and countries by means of telephone exchanges, 

 that we became acquainted with a machine or mechanical 

 system which was at all comparable to the central nervous 

 system of the human body. Before that comparison 

 became possible the exchange centres of the brain and 

 cord had to be unveiled and the nerve cables unravelled. 

 In order that we may understand how medical men 

 succeeded in unravelling the chief machinery of the brain 

 and spinal cord we must make a short journey in space 

 and time, so that we may watch some of the pioneers set 

 out on their labours. The inquiries of John Hunter, 

 made in London during the latter half of the eighteenth 

 century, provide us with a suitable point of departure. 

 To see him at work we must seek out Covent Garden is 

 the heart of London — not the built-over busy market of 

 to-day, but the open fashionable square which met the 

 eye of young John Hunter as he entered it on a late 

 September day of 1748, travel-stained from eleven days 

 spent on horseback in his journey from Scotland. He 

 seeks a house on the north side of the square where his 

 elder brother has established a school of anatomy, and 

 presently we find this rough sandy-haired Scot, a man of 

 twenty and accustomed only to farm life, assisting in his 

 brother's dissecting-room with great dexterity. We have 

 only to visit this room a few years later — in 1754 — to see 

 the kind of man he is developing into. He has a dis- 

 section of the nose in front of him, and is tracing out the 

 various nerves which end in its lining membrane. That 

 dissection is still preserved in the Museum of the Royal 

 College of Surgeons of England, but our interest must 

 remain centred on the fact that, at the time we now visit 

 him, Hunter is wondering why the same area of lining 

 membrane should receive nerves from two sources — from 



