84 TRANSPORTATION SYSTEM IN ANIMALS 



handicap is overcome by small fluid-containing spaces 

 among the tissues upon which every cell has a frontage. 

 Moreover, these spaces usually form a continuous sys- 

 tem that is supplied at definite points with food 

 materials and kept in action by a definite pump, 

 the heart, so that the wants of every cell are adequately 

 provided for. 



Cell Diffusion. — The dissolved food materials pass 

 into the cell by the process of diffusion, as described in 

 a previous chapter. Hence it follows that the cell mem- 

 brane must not only be thin and permeable to permit of 

 a rapid movement, but it must be of relatively great ex- 

 tent when compared to the enclosed protoplasmic mass. 

 This state of affairs exists only when the mass is small, 

 as will be seen from the following illustration: 



Cell Surface and Body Surface. — If we take a 

 cubic foot block of wood and saw it into cubic inch blocks 

 the external surface of these smaller masses is twelve 

 times greater than that of the original one. If we reduce 

 it all to sawdust the resultant surface is increased to a 

 much greater degree. The finer the subdivision the 

 greater the entire surface. 



Applying this principle to the body of a man, for ex- 

 ample, we find that the outer surface measures approxi- 

 mately sixteen square feet, a relatively small amount 

 when compared to the mass weighing 150 pounds. But 

 in reality this body, excluding much of the bones, body 

 fluids and certain other structures, is subdivided into an 

 inconceivably great number of cells. In the blood alone 

 it is estimated that there are 25,000.000,000,000 cells, and 

 it is safe to say that the entire body comprises at least 

 five times this number. If, therefore, the surface of the 

 blood cells has a superficial area of 3,800 square yards, 

 as has been estimated, it follows that the surfaces of all 

 the colls of the body measure not less than 170,000 square 

 feet, or more than 10,000 times greater than the body 

 surface. 



