242 THE ENGINES OF THE HUMAN BODY 



tebral — enter the skull and assist the carotids. The 

 spinal cord is provided with a succession of small arteries. 

 There is one curious feature of the blood circulation in 

 the central nerve machine. Being contained in a chamber 

 with rigid walls, and always quite full, every charge of 

 arterial blood which enters the chamber has to displace 

 an equal amount of venous blood. Thus each pump of 

 the heart always fills the arteries and empties the veins of 

 the brain at the same time. 



We know that the nerve operatives are particularly 

 sensitive to a lack of oxygen. They die the instant their 

 stock is exhausted, and at the most that stock will not 

 last them ten minutes once the outside supply is cut off". 

 Hence in people who have been submerged in water for 

 ten minutes and apparently drowned, although the heart 

 and lungs still retain life and can be revived, it is usually 

 the case that the nerve operatives or cells are already dead 

 and destroyed beyond recovery. The great need for 

 oxygen, perhaps, accounts for the liberal blood supply to 

 the brain, for we know that the nerve operatives consume, 

 even when they are writing books, a very small amount 

 of fuel. Even the kind of fuel they consume we do not 

 yet know. Their staple diet is not likely to be the rough 

 carbo-hydrate fuel which muscles turn into work. Per- 

 haps some day we shall be able even to estimate the 

 exact amount of fuel and oxygen which are consumed 

 during the elaboration of a great thought or the solution 

 of an intricate problem in mathematics. 



