INTRODUCTORY. ' 3 



menced, and to a certain extent carried out, by a more 

 or less complicated digestive apparatus. In any case, the 

 first part of assimilation consists in the melting down of 

 the food into a common nutritive fluid, which contains 

 certain organic compounds which existed in the food to 

 begin with, or were manufactured out of it in the process 

 of digestion. In the higher animals, the food is so acted 

 upon in the stomach and intestines that it forms a complex 

 fluid, Avhich is then "absorbed" or sucked up from the 

 alimentary canal, to form the " blood." The blood, there- 

 fore, is to be regarded as an organic fluid which is manufac- 

 tured out of the food which the animal eats. The blood 

 has dissolved in it the materials which are necessary for 

 making new tissues, and it has to be distributed to the 

 various parts of the body, so that each tissue may take from 

 it the substances which are requisite to repair its losses. 

 This is usually efi'ected by a distinct propulsive organ — 

 the "heart" — which drives the blood to all parts of the 

 body. 



What occurs, then, in any of the higher animals, is 

 readily understood. The various parts or tissues of the 

 body are gradually wearing away, and they require fresh 

 material for replacing their waste. This fresh material is 

 contained in the blood, and is derived from the food ; and 

 it is incessantly driven to the difi"erent parts of the body. 

 As the blood, therefore, circulates through each organ of 

 the body, that organ abstracts from its living current 

 the materials required to restore its losses and its waste. 



Not only does each organ of the body take from the 

 blood the materials requisite to repair its losses, but each 

 at the same time throws off into the blood the worn-out 

 and useless materials which have been produced by its 

 slow destruction and wearing away. The result of this is 

 that the blood very rapidly becomes impure, and gets 

 loaded with waste matter as it circulates through the 

 tissues and organs of the body. 



It follows from this that the blood has two processes 

 to undergo, if it is to remain in a healthy condition. In 

 the first place, the various tissues of the body are con- 



