A TRANSPORT SYSTEM 179 



of years of experimentation and invention on the part of 

 Nature. She has forgotten nothing — not even the pro- 

 vision of means for the overcoming of a block or obstruc- 

 tion. As a result of disease, a narrowing or constriction 

 may form at some point of the oesophagus — often just 

 behind where the windpipe divides into the two bronchi 

 — the passages to the lungs. When a contraction ring, 

 which is urging a bolus onwards, finds that its load has 

 stuck against such a constriction, it passes on and becomes 

 dissipated. Presently another ring appears, more vigorous 

 than its predecessor ; if it fails, another sets in and the 

 effort is redoubled. Nothing could be more persistent 

 than the attempts made by the muscle spindles ; the 

 mechanism which controls their contraction is almost 

 inexhaustible. We find, after a time of persistent effort, 

 that the muscular coats have increased in thickness and 

 in strength, the lubricating glands pour out their secre- 

 tion more freely and abundantly. The human machine is 

 one of these rare engines which develops a greater horse- 

 power automatically as its load is increased. Muscle 

 spindles have this power to a remarkable degree — more 

 than any other tissue of the human machine. Often the 

 nature of an oesophageal obstruction is such that their 

 best efforts are vain. 



Mention has already been made of the fact that, 

 when the thorax is transilluminated by X-rays, a bolus 

 can be seen to stop for a moment at the lower end of the 

 oesophagus before being gently ushered into the stomach. 

 That is because the passageway is there guarded and 

 kept shut by a sphincter mechanism — muscular lock- 

 gates, and as we shall meet several contrivances of this 

 kind at other junctions of the alimentary system, we shall 

 look at this one — the cardiac sphincter of the stomach — 

 in some detail. As seen with the naked eye (see fig. 40) 

 this sphincter appears to be a very simple structure ; it 

 is merely a short segment — a ring — of the inner muscular 

 coat of the oesophagus slightly strengthened. It differs, 

 however, from the rest of the circular coat by being 

 maintained constantly in a state of action or contraction. 



