198 MANUAL OF ANIMAL BIOLOGY 



One of the most interesting demonstrations of the functioning 

 of the vascular system is to be seen when a small portion of the 

 living interdigital web of the Frog's foot is observed under the 

 microscope. In such a demonstration, the characteristics of the 

 flow of blood through the arteries, capillaries, and veins can easily 

 be seen. Thus, it will be noted that the flow is most rapid through 

 the arteries, where a definite pulsation can be detected which, 

 of course, results from increased pressure due to the contraction 

 of the ventricle. The flow in the veins is much slower and no 

 pulsation is noted. The capillaries will be seen as very tiny ves- 

 sels through which the red corpuscles have to pass in single file. 

 All in all, an amazing impression of the vitality and omnipresence 

 of the vascular system is presented. 



Functional. Our previous consideration of the vascular system 

 has shown that it is responsible for the transportation, to and from 

 the cells, of all essential materials. In order to get the details 

 clearly in mind, it will be well to enumerate the chief items trans- 

 ported, the methods of transportation, and the relations to other 

 life functions. (W. f. 126.) 



Nutrition. The soluble food materials absorbed from the ali- 

 mentary canal by the mucosa cells are carried in solution in the 

 blood plasma and, except for the fats, go first to the liver through 

 the portal vein, where the digested carbohydrates are temporarily 

 removed and changed into glycogen by the hepatic cells. The 

 latter is replaced in the blood stream from time to time in such 

 amounts as are needed to supply the demands of the cells for 

 'quick energy.' Various amino acids, resulting from the digestion 

 of the proteins, are present in the blood plasma, and each cell 

 selects the particular ones necessary for the manufacture of its 

 specific protoplasm. Fats are largely absorbed by the lymphat- 

 ics (lacteals) in the villi of the intestine and pass through 

 the thoracic duct and then into the blood stream. (W. fs. 114, 

 115.) 



Respiration. The blood of the Vertebrate transports the essen- 

 tial and continuous supply of oxygen obtained from gills, lungs, or 

 skin, almost entirely in the red corpuscles, where, as previously 

 noted, it combines with the hemoglobin to form oxyhemoglobin — 

 an unstable compound which lasts only until the oxygen-deficient 

 cells are reached. The oxyhemoglobin formed in the red corpuscles 

 gives arterial blood its bright red color. The carbon dioxide ex- 



