358 OF THE CIRCULATION OF BLOOD. 



CHAPTER IX. 



OF THE CIRCULATION OF BLOOD. 



I. Of the Circulation in General. 



475. THE Circulation of nutritive fluid through the body has for its object, 

 to convey to every part of the organism the materials for its growth and reno- 

 vation, as well as the supply of oxygen which is required for the performance 

 of its vital actions : and also to carry off the particles which are set free by the 

 disintegration or waste of the system, and which are to be removed from it by 

 excreting processes. Of these processes, the one most constantly in operation 

 as well as most necessary for the maintenance of the purity of the blood, is the 

 extrication of carbonic acid from the respiratory organs ; and this is made sub- 

 servient to the introduction of oxygen into the system. The extent, therefore, 

 to which a circulating apparatus is developed in the animal kingdom, is partly 

 dependent upon the degree in which the function of nutritive absorption is 

 limited to one part of the body, and partly upon the arrangement of the excret- 

 ing surfaces, and especially of the respiratory apparatus. Where the digestive 

 cavity itself extends through the whole system, so that every part can absorb 

 at once from its parietes, and where the whole external surface is adapted, by 

 its softness and permeability, to expose the fluids of the body to the aerating 

 medium around, there is no necessity for any transmission of fluid from one 

 part to another ; and accordingly, in the lowest animals, which are thus formed, 

 no true circulation exists. Again, in the insect tribes, in whose bodies the 

 absorption of fluid can only take place at fixed points, there is a circulation, for 

 the purpose of transmitting the absorbed matter to the remote parts of the body ; 

 but, as every part of the interior is permeated by air, the second of the above- 

 named purpose is already answered ; and the circuit of the blood through the 

 vessels, therefore, is not accomplished with the energy and activity which, 

 from the vigorous movements performed by these little beings, might have been 

 supposed necessary. On the other hand, in the Mollusca, the absorption of 

 fluid is liraited, and the respiratory action equally so ; and among these we find 

 the circulation performed with nearly as much vigour as it is in the Vertebrata. 

 * 476. In Man, as in other Vertebrated animals, there is a regular and con- 

 tinuous movement of the nutritive fluid through the vascular system; and 

 upon the maintenance of this, the activity of all parts of the organism is de- 

 pendent. The course of the Blood may be likened to the figure 8 ; for there 

 are two distinct circles of vessels through which it is transmitted; and the 

 Heart is placed at the junction of these. The Systemic and Pulmonary circu- 

 lations are entirely separate, and might be said to have distinct hearts ; for the 

 left and right sides of the heart, which are respectively appropriated to these, 

 have no direct communication with each other, (in the perfect adult condition, 

 at least,) and are merely brought together for economy of material. At an 

 early period of fetal life, as in the permanent state of the Dugong, the heart 

 is so deeply cleft, from the apex towards the base, as almost to give the idea 

 of two separate organs. Each system has its own set.of arteries, or efferent 

 vessels, and veins or afferent trunks; thse communicate at their central 



