34 DESCRIPTION OF THE SYSTEMIC, PULMONARY, AND PORTAL CIRCULATION. 



chanical considerations might, perhaps, lead us to infer that these ancient views are not 

 altogether correct. There does not seem much consonance between mitral, tricuspid, 

 and semilunar valves, and affections or passions, or the responsibilities for good and evil. 

 A pump may be a very curious and ingenious piece of mechanism, but surely we can- 

 not regard it as a morally accountable agent. 



105. For such reasons, therefore, modern physiologists are disposed to cast aside al- 

 together these ancient views, and look upon the heart as an ordinary but beautiful speci- 

 men of hydraulic contrivance, in the same way as they look upon the eye as an opti- 

 cal apparatus. 



106. In man there are three prominent varieties of circulation : the Systemic, the 

 Pulmonary, and the Portal. It is necessary that we should understand the nature of 

 each. 



1st. THE SYSTEMIC. Arterial blood, which has been brought from the lungs by the 

 pulmonary veins into the left auricle, is forced by its contraction into the left ventricle, 

 the contraction of which drives it into the aorta. By this it is distributed to all parts 

 of the system, the arrangement of arterial tubes becoming smaller and smaller in diam- 

 eter until they degenerate into mere capillaries, from which it finds its way into the 

 ascending and descending vena cava, the terminal tubelets of which collect it, and k is 

 returned to the heart by the right auricle during its dilatation. In the systemic circu- 

 lation, therefore, the blood leaves the left ventricle as arterial blood, is finally distrib- 

 uted by the capillary arteries, undergoes, on its passage into the capillary veins, a chem- 

 ical change, gives off oxygen, from being bright red it turns dark, and, becoming venous 

 blood, is then brought back to the heart, entering its right auricle. 



2d. THE PULMONARY. The venous blood, thus brought to the right auricle, is forced 

 into the right ventricle when the auricle contracts. A similar contraction of the right 

 ventricle now forces it into the pulmonary arteries, by which it is distributed upon the 

 air-cells of the lungs ; here a chemical change takes place ; from being dark venous 

 blood, it becomes bright red arterial, and, being collected by the pulmonary veins, re- 

 turns, finally, to the left auricle of the heart. In the pulmonary circulation, therefore, 

 the blood leaves the right ventricle for the lungs, undergoes in them a change, giving 

 off carbonic acid, and then returns to the left auricle. 



3d. THE PORTAL. The portal vein collecting blood from the chylopoietic viscera, 

 distributes it to the liver. This blood there mingles with that which has been derived 

 from the hepatic artery, and which has already been deoxydized in that organ. From 

 this portal blood, bile is secreted and passed into the biliary tubes. The changed blood 

 is now collected by the ramifications of the hepatic veins, along which it is transmitted 

 to the ascending vena cava. The portal circulation is, therefore, apparently not con- 

 nected with any central organ of impulse. It commences in a capillary system, and 

 terminates in a capillary system. There is no hydraulic mechanism for the purpose 

 of determining a current. 



107. Of these different varieties of circulation, let us now select one, and show that 

 the principles we have employed in describing the flow of sap both upward and down- 

 ward, apply here also. Let us, therefore, confine for the present our considerations to 



