CIRCULATORY MEDIA 163 



moving along with the current. The blood platelets, which are small 

 portions of protoplasm derived from the cytoplasm of other cells, may 

 represent food carriers in the mammal blood. Some lower animals 

 probably carry most of their food in the blood as a solution. The food 

 material may bear some physiological relation to the blood, which must 

 maintain certain conditions in order to properly function. 



The blood usually carries the oxygen supply from the respiratory 

 surfaces to the tissues which use it. The oxygen is obtained by the blood 

 at these places by the chemical affinity of certain of its constituent sub- 

 stances for oxygen when they lack a certain proportion of it and are in 

 the presence of any substance that contains more than a certain propor- 

 tion of it in the free state. These substances also give it up to the tissues 

 with which they come in contact and which need it. 



One of the most prominent of these oxygen-bearing materials in the 

 blood is hemoglobin , which exists in the red corpuscles of the vertebrates 

 and free in the blood plasma of some invertebrates. Some of the 

 invertebrates have other substances with the same unstable affinities 

 for oxygen that haemoglobin has. There are several such substances. 

 They have other colors and are probably not as efficient as haemaglobm. 

 That of some mollusks is called hcemocyanine. Sometimes the blood 

 does not have the carrying of oxygen to do, the fresh air being conducted 

 all over the body by fine air tubes that bring it directly into contact with 

 the tissues. The insects show such a condition. 



The carrying of carbon dioxide, as a waste material from the tissues to 

 the respiratory surfaces for discharge, is also a duty of the blood in all 

 forms of animals except the insects, where it is discharged directly into 

 the air tubes. It is carried in the blood as a gas in solution. 



The uric acid and its compounds are probably always carried in solu- 

 tion by the blood, which always has a certain percentage of them in its 

 body. The blood is being constantly relieved of some of this burden 

 when it passes, in parts of its course, regions where the outer surface of 

 the vessels is lined by an epithelium that can extract this harmful mat- 

 ter from the blood and pass it into spaces that communicate with the 

 exterior for its discharge. 



The coagulation of the blood fluid for the purpose of closing any 

 accidental break in the vessels is brought about by the formation of 

 threads of fibrin in the blood of vertebrates or of colloid masses in the 

 blood of some invertebrates, or even by the gathering together of amoeboid 

 corpuscles which intertwine their processes together to form an obstruc- 

 tion to the escaping blood in still other of the lower animals. These 

 conditions occur automatically in cases where the animal is in health, 

 and the direct stimulation which brings clotting to pass, seems to be the 

 exposure to the air and the arresting of the blood stream, because this 



