INTRODUCTION. 9 



activity of which is similarly governed by the nervous system, and 

 hence the flow of blood to this part or that part is regulated 

 according to the needs of the part. 



§ 12. The above slight sketch will perhaps suffice to shew 

 not only how numerous but how varied are the problems with 

 which Physiology has to deal. 



In the first place there are what may be called general prob- 

 lems, such as, How the food after its preparation and elaboration 

 into blood is built up into the living substance of the several 

 tissues ? How the living substance breaks down into the dead 

 waste ? How the building up and breaking down differ in the 

 different tissues in such a way that energy is set free in different 

 modes, — the muscular tissue contracting, the nervous tissue thrill- 

 ing with a nervous impulse, the secreting tissue doing chemical 

 work, and the like ? To these general questions the answers which 

 we can at present give can hardly be called answers at all. 



In the second place there are what may be called special 

 problems, such as, What are the various steps by which the blood 

 is kept replenished with food and oxygen, and kept free from an 

 accumulation of waste, and how is the activity of the digestive, 

 respiratory, and excretory organs, which effect this, regulated and 

 adapted to the stress of circumstances ? What are the details 

 of the working of the vascular mechanism by which each and 

 every tissue is forever bathed with fresh blood, and how is that 

 working delicately adapted to all the varied changes of the body ? 

 And, compared with which all other special problems are insignifi- 

 cant and preparatory only, How do nervous impulses so flit to and 

 fro within the nervous system as to issue in the movements which 

 make up what we sometimes call the life of man ? It is to these 

 special problems that we must chiefly confine our attention, and 

 we may fitly begin with a study of the blood. 



