8 TISSUES AND ORGANS. 



with raw food, they are not required to carry out the necessary 

 transformations of their immediate waste matters. The whole of 

 the rest of the body is engaged (1) in so preparing the raw food, 

 and so bringing it to the nervous and muscular tissues that these 

 may build it up into their own substance with the least trouble, 

 and (2) in receiving the waste matters which arise in muscular 

 and nervous tissues, and preparing them for rapid and easy 

 ejection from the body. 



Thus to certain tissues, which we may speak of broadly as 

 ' tissues of digestion,' is allotted the duty of acting on the food and 

 preparing it for the use of the muscular and nervous tissues ; and 

 to other tissues, which we may speak of as " tissues of excretion," is 

 allotted the duty of clearing the body from the waste matters 

 generated by the muscular and nervous tissues. 



10. These tissues are for the most part arranged in machines 

 or mechanisms called organs, and the working of these organs in- 

 volves movement. The movements of these organs are carried out, 

 like the other movements of the body, chiefly by means of muscular 

 tissue governed by nervous tissue. Hence we may make a dis- 

 tinction between the muscles which are concerned in producing an 

 effect on the world outside man's body, the muscles by which 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 con- 

 cerned 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 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 con- 

 duct 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, visiting 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 supply 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 



