CHAPTER IX 

 THE TRANSPORTATION SYSTEM IN ANIMALS 



Conditions Favoring Cell Activity. — Every living 

 cell resembles a tiny laboratory whose structure deter- 

 mines the nature of the processes it carries on. The work 

 done by a nerve cell is different from that performed by 

 a muscle fiber and both differ from the operations of an 

 amoeba. Yet, however great their structural differences 

 and their modes of operation, all cells agree in requiring 

 certain conditions for their perfect operation. In the 

 first place the temperature must be above freezing and 

 below 120° F. in most instances. And, what is of the 

 utmost importance, in connection with the subject matter 

 of the present chapter, every active cell must be bathed 

 to a greater or less extent by a liquid. This water front- 

 age serves two purposes. It maintains the proper degree 

 of fluidity of the protoplasm, and, containing as it does, 

 food substances, it acts as a storehouse from which the 

 cell draws its supply and into which it discharges its 

 wastes. 



Need of Transportation. — Not only must every cell 

 have this water frontage from which it withdraws its 

 necessary food, but it is evident that this fluid must 

 be renewed or shifted in order to provide for new supplies. 

 Among the smaller aquatic animals, which consist of a 

 single cell or of many cells located at or near the surface 

 of the body, the movement of the water or of the body 

 in the water is all that is necessary in this respect. 

 Among the higher animals on the other hand, many of 

 the cells are buried far beneath the body surface and are 

 thus remote from the immediate source of supplies. This 



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