THE METABOLIC MACHINERY OK ANIMALS 303 



numbers of one sort or the other of these blood eells are produced. 

 The blood platelets are now generally beheved to play an important 

 role in the clotting of blood. 



In the web of a frog's foot the blood may be seen rushing along 

 through relatively large vessels which break down into smaller ones 

 until reaching the capillaries, through which the corpuscles slide in 

 single file at a much slower gait. It is here that oxygen and food 

 diffuse by osmosis to the surrounding lymph and so reach the tis- 

 sues. Under the microscope the blood appears to be traveling at a 

 headlong pace, due to the fact that this instrument magnifies only 

 space without reference to time. The pace of the corpuscles quick- 

 ens again as they reach the larger venules which, after anastomos- 

 ing, ultimately lead to the heart as veiris. Two interesting facts 

 might be mentioned here, one dealing with the capillaries and the 

 other with blood. Dr. Krogh, a Nobel prize winner from Denmark, 

 says that if an average human being was selected and all of his capil- 

 laries were opened up and spread out flat, their total area would nearly 

 cover that encompassed by an average city block. The other fact 

 centers about the numbers of corpuscles present, of which various 

 estimates have been made. In normal women and men there should 

 be 4,500,000 to 5,000,000 red corpuscles (erythrocytes) per cubic mil- 

 limeter of blood, while somewhere between 5000 and 10,000, nor- 

 mally about 7500 white corpuscles (leucocytes), is considered an 

 average count. Red corpuscles vary in number with altitude, a 

 greater number being necessary in high altitudes where less oxygen is 

 present in the atmosphere and, consequently, greater numbers are 

 needed to transport the amount of air necessary for life. 



The plasma of the blood also contains a great variety of protective 

 substances which are known under the general heading of antibodies. 

 They are induced by bacteria and other parasites which, acting as 

 foreign proteins, stimulate some living body cells to manufacture them 

 (see page 626) and turn their protective substances loose into the 

 blood stream. 



The Lymph 



Even though capillaries are distributed widely, each is surrounded 

 by narrow lymph spaces, that are filled with plasma and white 

 corpuscles, the latter being mostly lymphocytes. Lymph is concerned 

 with the transportation of food, oxygen, and other substances neces- 

 sary for the successful metabolism of the organism. It is lymph which 



