162 HISTOLOGY 



FRANZ. " Uber die Struktur des Herzen und die Entstehung von Blutzellen bei Spin- 



nen," Zool. Anz., Band XXVI. 

 BERGH, R. S. "Uber den bauder Gefasse bei den Anneliden," Anat. Hefte, Band XIV 



und Band XV. 



THE CIRCULATING MEDIA: BLOOD 



Being mostly a fluid, the study of the blood from a physiological and 

 a chemical point of a view would be more enlightening than to find out 

 what can be known of it from its histological structure under the micro- 

 scope. Physiological study of this fluid, assisted by chemical studies 

 and certain structural studies, have taught us that some of its functions 

 are, roughly speaking, the following: 



The primary transportation of nutriment from the digestive tract 

 and its distribution in the tissues; the secondary transportation of food 

 materials to and from storage in the liver and elsewhere ; the transporta- 

 tion of oxygen from the organs of respiration to the tissues; and lastly, 

 as a result of the use of these materials, the carrying of the products of 

 combustion, carbon dioxide, and urea, to the respiration organs and the 

 nephridia respectively, to be cast out as waste matter. More mechanical 

 in its operation is the function of providing transportation, by floating, 

 for many moving cells, the blood corpuscles, that have duties to perform 

 in other parts of the body, and lastly, the blood has the power of coagu- 

 lating into a more or less hard mass, for the purpose of preventing hem- 

 orrhages when the circulatory system is cut or injured at any point. 



In its origin the blood must be looked upon as derived from the 

 mesenchymal tissue, part of whose cells formed its walls and others the 

 blood cells or corpuscles. By some investigators it is thought that the 

 plasma as well as the blood cells are derived from the excavated interiors 

 of chains of these cells, called the vaso-formalive cells. It is possible that 

 the blood elements are derived from certain of the mesenchyme cells 

 that lay between the others, which became the walls of the blood channels. 



The blood of different animals varies much as to which of the various 

 functions it performs and how it performs them. We shall take up some 

 of these functions and discuss the method of executing them in the vari- 

 ous ways in different bloods, particularly as to the structural features 

 employed. 



The carrying of food materials (other than oxygen) is a principal 

 function of the blood. It can only do this when the foods have been 

 properly prepared by the digestive processes. Otherwise the food mat- 

 ter might seriously interfere with the performance of other duties. The 

 degree of this preparation varies in different animals. Most foods are 

 carried as a solution. Some are transported as an emulsion, and some- 

 times the blood cells carry solids, taking them into their cytoplasm and 



