INTRODUCTION. 23 



man does his work in the world, and the muscles which are concerned in 

 carrying out the movements of the internal organs. And we may similarly 

 make a distinction between the nervous tissue concerned in carrying out the 

 external work of the body and that concerned in regulating the movements 

 and, as we shall see, the general conduct of the internal organs. But these 

 two classes of muscular and nervous tissue, though distinct in work and, as 

 we shall see, often different in structure, are not separated or isolated. On 

 the contrary, while it is the main duty of the nervous tissue as a whole, the 

 nervous system as we may call it, to carry out, by means of nervous impulses 

 passing hither and thither, what may be spoken of as the work of man, and 

 in this sense it is the master tissue, it also serves as a bond of union between 

 itself and the muscles doing external work on the one hand, and the organs 

 of digestion or excretion on the other, so that the activity and conduct of 

 the latter may be adequately adapted to the needs of the former. 



11. Lastly, the food prepared and elaborated by the digestive organs 

 is carried and presented to the muscular and nervous tissues in the form of 

 a complex fluid known as blood, which, driven by means of a complicated 

 mechanism known as the vascular system, circulates all over the body, visit- 

 ing in turn all the tissues of the body, and by a special arrangement known 

 as the respiratory mechanism, carrying in itself to the several tissues a sup- 

 ply of oxygen as well as of food more properly so called. 



The motive power of this vascular system is supplied, as in the case of 

 the digestive system, by means of muscular tissue, the 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 show 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 problems, 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 thrilling 

 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 maybe 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 various changes of the body ? And, compared with which 

 all other special problems are insignificant 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. 



