80 HUMAN PHYSIOLOGY 



CHAPTER III 



THE STORY OP DIGESTION 



WHAT DIGESTION Is. A common notion regarding 

 the nature of digestion is that which defines it as 

 the work of converting food into blood. This is an 

 entirely erroneous definition. Food is not con- 

 verted into blood, but is changed by digestion into 

 such a form that it can be added to the blood and 

 be readily incorporated with that fluid with which in 

 due season it becomes identical. It might truly be 

 said that in many respects the food we eat presents 

 a certain chemical resemblance to the blood itself, 

 but whilst digestion involves certain actions of a 

 physical kind, it is also represented by a series of 

 chemical aspects wherein the elements of the food 

 are recombined after being in many cases split up 

 or dissociated, and thus made available to renew 

 and repair the vital fluid. The blood, a topic to be 

 hereafter treated, may be regarded as the common 

 currency of the body out of which all the tissues of 

 the body take precisely the nourishment which is 

 adapted to renew and repair their vitality, so that 

 in one sense the blood has been well described as 

 being a kind of fluid epitome of the body itself. 



THE DIGESTIVE SYSTEM. It is possible to form a 

 simple, and at the same time correct, idea of the 

 nature of any digestive system ranging from that of 

 the lowest animal, in which such a system appears, 

 onwards to that of man himself. Any digestive 



