OF THE CIRCULATION IN GENERAL. 



541 



is already answered ; and the circuit of the blood through the vessels, there- 

 fore, is not accomplished with the energy and activity which, from the vigor- 

 ous movements performed by these little beings, might have been supposed 

 necessary. On the other hand, among the MoUusca, in which the absorption 

 of fluid and the respiratory action are alike limited, we find the circulating 

 apparatus almost as extensive, and the movement of blood as vigorous, as it is 

 in the lower Vertebrata. It is in those animals, in which there is the greatest 

 activity in the other functions, which live, in fact, the fastest, that the Cir- 

 culation is most energetic ; thus the rapid and energetic movement of the 

 blood in Birds contrasts most strongly with its slow and feeble propulsion in 

 Reptiles. The movement may vary considerably, however, in the same ani- 

 mal at different times, according to its state of repose or activity ; and in dif- 

 ferent organs of the same animal, according to the energy with which their 

 functions are being respectively performed. 



710. 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 dependent. 

 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 circulations 

 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 foetal 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; these communicate at their central ex- 

 tremity by the Heart ; and at their peripheral extremity by the Capillary ves- 

 sels, which are nothing else than the minutest ramifications of the two systems, 

 inosculating into a plexus ( 219). 



Fig. 211. 



Web of Frog's foot, stretching between two toes, magnified 3 diam. ; showing the blood-vessels, and 



their anastomoses : 1,1, veins; 2, 2, 2, arteries. 



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