ANCIENT THEORY OF THE CIRCULATION. 33 



CHAPTER III. 



ON THE MECHANICAL CAUSE OF THE CIRCULATION OF THE BLOOD. 



CONTENTS : Ancient Theory. Description of the Systemic, Pulmonary, and Portal Cir- 

 culation. General Law of Movement. Capillary Relations of Arterial and Venous 

 Blood to the Tissues. The Systemic Circulation is due to the Deoxydation of Arte- 

 rial Blood, and its Direction is therefore from the Artery to the Vein. 



Pulmonary Circulation. Capillary Relations of Arterial and Venous Blood to Atmo- 

 spheric Oxygen. Pulmonary Circulation is due to the Oxydation of Venous Blood, 

 and its Direction is therefore from the Venous to the Arterial Side. Uses and Action 

 of the Heart. 



Porta/ Circulation. Capillary Relations of Arterial, Portal, and Venous Blood to the 

 Lin/: Three Sources of Force in conducting the Portal Circulation. 



Action in Asphyxia. Case of obstructed Trachea. 



102. LET us now proceed to inquire how the physical principles which have guided 

 us, in the preceding chapter, in determining the cause of the flow of sap, apply to the 

 more interesting case of the circulation of the blood in the higher animals. 



103. The popular explanation which is given of the circulation of the blood in man, 

 refers to the heart as the prime mover of the mechanism. This central organ of im- 

 pulse is devoted to a double purpose. It has to throw the blood through the channel 

 of the arteries to every part of the system, and, receiving it back again by the veins, 

 has to throw it to the lungs, in which it must be submitted to the vivifying influence 

 of the air before it can again be restored to the system to be used for the general pur- 

 poses of the economy. In order to enable it to discharge this task, it is furnished with 

 an appropriate valvular and tubular arrangement, and, at specific periods, contracts and 

 dilates, for the purpose of ejecting or sucking up the circulating liquid. In the opin- 

 ion of the older physiologists, these periodic motions take place either directly, for the 

 reason that the heart is alive, or, as some of them have supposed, through the myste- 

 rious agency of the cerebro-spinal axis. 



104. The blood, thus alternately driven from and drawn to this centre of action, is, 

 in this view, a liv ing fluid, possessed of a great many extraordinary properties. A por- 

 tion of it extracted from the system by any of the ordinary processes of phlebotomy, 

 soon coagulates and dies. In what this change consists, this distinction between the 

 living and dead blood, does not so plainly appear. Moreover, the ancient physiologists 

 imputed a variety of other equally important offices to the heart; they regarded it as the 

 seat of the passions, such as love, and held it accountable for the various deeds in which its 

 possessor was concerned, a moral accountability for good and evil. These philosophical 

 doctrines have, to a certain extent, become interwoven in common speech, and we now 

 often use them without attaching any strict signification to them. Anatomical and rne- 



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