INTRODUCTION 3 



of the movement of our own blood. Yet over four litres 

 are leaving each ventricle per minute and passing through 

 the aorta with a velocity of about eighteen centimetres 

 per second. 



The rapidity with which the blood flows and its indis- 

 criminate distribution among the tissues have certain 

 important results. Any abnormaUty in the metabohsm 

 of one tissue immediately affects, through the blood, 

 the whole body. While the circulation is free there can 

 be no locahsation of a substance soluble in the blood. 

 This freedom of the circulation is made use of for the purpose 

 of co-ordinating the activities of the different organs. In 

 the first place, the accumulation of normal products of 

 metabohsm leads to a series of changes in other organs. 

 In the second place, certain organs have become specialised 

 solely to produce substances which quicken or retard some 

 general bodily function. These substances are known as 

 internal secretions or hormones. 



This chemical method of co-ordination has at once an 

 advantage and a disadvantage. .The advantage lies in the 

 nicety of adjustment which is possible, due partly to the 

 potency of the chemical substance formed, partly to the sen- 

 sitiveness of the organ upon which it acts. The respira- 

 tory centre, for instance, is a far more dehcate indicator 

 of the reaction of the blood than any known chemical 

 reagent. Again, adrenahn exerts its effects in the strength 

 of one part in a million. The disadvantage of the chemical 

 method is the time which it takes to work its effects. 

 Rapid as the circulation is, it is not sufficiently rapid for 

 the proper co-ordinated response where time is an important 

 factor. 



For rapid co-ordination Nature makes use of the irrita- 

 bihty of protoplasm. The nerve cells, some of them cells 

 of great length, are specially adapted to conduct disturb- 

 ances arising in one part to different parts of the body. 

 The grouping of nerve cells to form the central nervous 

 system is for the purpose of effecting rapidly, in response 



