530 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



resistance, evidently to be* explained by the fact that as the arteries 

 divide their total calibre increases. If the mean pressure in the artery 

 is measured at different times it will be found to be subject to very great 

 variations, from causes which will subsequently receive attention. 



The average blood pressure of mammals is by no means dependent 

 upon the animal's size. In all cases the arterial blood pressure may be 

 stated as exceeding the atmospheric pressure and varying between one 

 hundred and two hundred millimeters of mercury. 



The following table gives various estimates of the mean arterial 

 pressure in different animals (Volkmann) : 



The facts thus reached experimentally as to the gradual decrease in 

 pressure from the arteries to the commencement of the veins and from 

 there to the larger venous trunks completely explains the constant cur- 

 rent of blood from the arteries to the veins. The blood, therefore, moves 

 in a circle from the heart to the arteries, through the capillaries to the 

 veins, and from the veins to the heart, to again enter the arterial system. 



The Velocity of the Blood. From the fact that the arteries as they 

 pass into the capillaries increase immensely in area, and as the capillaries 

 pass into the veins a corresponding decrease is found, it is to be expected 

 that the velocity of the blood-current will be greatest in the vessels near 

 the heart. As the blood leaves the heart to pass into the aorta the 

 velocity of the current is at its maximum ; it then gradually decreases as 

 the capillaries are reached, then undergoes a sudden retardation, and again, 

 as the blood is collected from the capillaries in the veins, the current 

 moves with an increasing velocity as the right side of the heart is ap- 

 proached. No absolute figures can be given as representing the normal 



