172 THE ENGINES OF THE HUMAN BODY 



the ball of a rubber syringe with the fingers and palm of 

 our hand. The manner in which the fingers close on the 

 rubber ball illustrates the mode in which the muscular 

 wall of the left ventricle expels its charge of blood. The 

 alimentary canal of the human body is fitted from be- 

 ginning to end to serve as a muscular pump. Every- 

 where its tube or pipe is clothed with two layers or strata 

 of muscle arranged so as to be able to propel the contents 

 of the tube from point to point along the whole course. 

 Thus the engines which are used in the transport of food 

 along the alimentary canal are microscopic in size and are 

 stationary in nature, being built within the wall of the 

 tube. They are muscular engines of a type we have not 

 seen before. They are not shaped like cylinders, but 

 taper at each end like a spindle (see fig. 38). They are 

 tightly packed together, side by side, their tapering ends 

 overlapping and interdigitating. The total number of 

 muscle fibres or spindles involved in transporting a single 

 bolus of food along the 28 feet of alimentary tract is un- 

 countable, for although the total length of each engine 

 spindle amounts to yg-th of an inch, their width is only 

 5 ^jth. Taking an average piece of bowel 1 inch in length, 

 with a circumference of 3 inches, a piece in which the two 

 muscular coats make up a combined thickness of Jth of 

 an inch, an estimate will show that in this short segment 

 there are about two millions of these tapering muscle 

 fibres or engines. Thus for the transport of even a single 

 bolus through such a short segment of the alimentary 

 tract two millions of engines have to be set in motion, 

 in a definitely co-ordinated sequence. Each has to be 

 started and stopped at a certain phase of the movement. 

 The effect is simple, but the underlying mechanism is 

 highly complex. Is there any wonder that such a trans- 

 port system should break down occasionally, and that its 

 repair should tax the ingenuity of the physician to the 

 utmost ? 



The exact manner in which these muscular fibres are 

 co-ordinated in their task of propelling the meals we eat 

 along the alimentary canal we shall see as we proceed ; 



