THE MASTER CONTRIVANCE 241 



from the canal of the backbone by special windows 

 placed between the vertebrae. Opposite the arm and 

 the leg these nerve cables are exceptionally large, because 

 the nerve traffic between the limbs and the central nerve 

 machine is a particularly busy one. Five pairs of these 

 spinal nerves enter the upper limbs, while the lower 

 limbs receive seven pairs of the largest size. 



Within the skull twelve pairs of cranial nerves arise 

 from the brain stem (fig. 45, B). The first pair go to 

 the nose, the second pair to the eyes ; both of these are 

 directly connected with the cerebrum. The eighth, 

 springing from the medulla, supply the ear ; the ninth 

 contain the fibres which arise in the taste buds of the 

 mouth ; the tenth, or vagus pair, spring from the medulla 

 and descend within the thorax and abdomen, where they 

 bring the lungs, heart, and stomach into communication 

 with the small masses of grey matter in the medullary 

 stations which serve as exchange centres for nerve "calls." 

 The other cranial nerves have to do with the muscles 

 of the eyeball, face, and neck, and with the nerve supply 

 of the skin of the face. Thus each half of the body is 

 linked to the central nerve machine by forty-three nerve 

 cables, varying greatly in size, but all of them issuing 

 in orderly sequence from above downwards. No matter 

 how small the part of the body damaged, it is certain 

 to have nerves and thus have the means of communicating 

 with the brain. 



The central nerve machine, then, is constructed of two 

 elements — microscopic living operatives and living wires 

 — more delicate than the finest threads of a spider's web. 

 Both operatives and wires, being alive, must be supplied 

 with fuel and oxygen. They apparently need a plentiful 

 supply, for the carotid arteries which pierce the base of 

 the skull to carry blood to the greater part of the brain 

 are large vessels. They send relatively small branches to 

 supply the deep masses of grey matter, but the cortical 

 areas are permeated with a rich capillary field. The 

 machinery which supports our consciousness needs a 

 liberal blood supply. Two smaller arteries — the ver- 



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