CHAPTER XII 

 TISSUES OF CIRCULATION: GENERAL CONSIDERATIONS 



WE have seen that the body of an organism, when it is not a single 

 cell, is a mass of cells closely applied to each other in a manner designed 

 to shut out most outer conditions and all foreign materials except such 

 as may be admitted by design. We also know that all of the cells of this 

 body, while living, must be taking in the materials used to sustain life, 

 while at the same time they are throwing out and discarding certain other 

 materials that are no longer wanted. The first of these materials, which 

 we may call the food materials, must be brought into the body from the 

 outside. They are taken in, through or between the cells that cover the 

 body surface, or some particular part of this surface. This is very evident, 

 as they could not enter in any other way. And it is also true that these 

 surface cells must act as the medium through which the waste matter 

 spoken of above is cast out. This thought serves to impress us with the 

 importance of surface as a factor in the operations of the animal body 

 and of the constant transfer of material that must go on in all its parts. 



More important for present discussion, but not to be separated from 

 the above ideas, is the fact that the materials, once inside the body, must 

 be passed from cell to cell, must be distributed and must be gathered. 

 This means that they are constantly transported. Such transportation 

 in its simplest form is a physiological process of the cytoplasm or a physi- 

 cal process of osmosis or both. Lastly, it may happen that the materials 

 pass between the cells instead of through them, and they may be assisted 

 on their way by fluids that carry them in solution or otherwise. Such a 

 system of passages between the cells constitutes a circulatory system 

 for the distribution of materials, and the fluid used to carry the 'matter 

 is called a circulatory fluid or blood. 



It is true that if the body is so small that the materials can be easily 

 passed from cell to cell through its mass, that there is no need for a circu- 

 latory system. And when the mass of the body becomes too great for 

 an effective distribution from the outer surface, it is still possible to both 

 increase the amount of surface and to make the distribution effective by 

 a series of invaginations. 



In order to understand the comparative value of such a method and 

 of an internal transporting or circulatory system, we shall examine a 



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