SECTION VII. 

 The Circulation of the Blood. 



It has been seen that digestion is the preparation of food for ab- 

 sorption ; absorption is the process by ■which the results of digestion 

 reach the interior of the blood-vessels ; but that the blood, which by 

 means of absorption has thus received the nutritive principles of the food, 

 may satisfactorily meet the nutritive wants of the different tissues of 

 the body, it must be in constant motion. The circulation of the blood is, 

 therefore, that function by means of which the nutritive materials sup- 

 plied by absorption are distributed to the economy after being subjected 

 to aeration, and by which the refuse and effete materials are carried where 

 they may be excreted. 



1. General Yiew of the Organs of Circulation. — Circulation is an 

 organic function, being present in both the animal and vegetable king- 

 doms. 



In the simplest forms of life, both animal and vegetable, in which 

 absorption takes place by imbibition from the entire external surface, 

 no special circulatory apparatus is required. It is only when certain 

 tissues become specialized organs for absorption and others for aeration 

 that a necessity arises for some apparatus by which the materials ab- 

 sorbed are conveyed from the point of absorption to the respiratory 

 organs and to the sj-stem at large. The development of the circulatory 

 organs is, therefore, proportional to the degree in which absorption and 

 respiration are limited to special tissues. 



As might be expected from the definition of the circulation, in 

 the lowest animals, as in plants, in which absorption takes place from 

 the entire external surface, there exists no apparatus for carrying 

 on a circulation of fluid, the contractile vesicles seen in many of the 

 protozoa having, probably, rather a respiratory than a circulatory 

 function ; it is only when the digestive organs become highly specialized 

 that a circulatory apparatus appears. Thus, in the coelenterata the 

 somatic cavity is in free communication with the digestive cavit}^ and 

 with the exterior, and the fluid which it contains, representing the blood 

 of higher orders, is moved by the contractions of the entire body and by 

 the vibration of cilia lining the somatic cavity, there being no indication 

 of either a heart or a vascular system. In the turbellaria, trematoda, 

 and cestoidea the lacunae of the mesoderm and the interstitial fluid of 



(491) 



