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THE AMERICAN NATURALIST [Vol. XLIX 



ous system and the oxygen tension in the blood. We 

 may next consider some of these subsidiary mechanisms. 



2. The Cardiac and Vascidar Mechanisms 

 The mechanism for the distribution of the blood is in 

 itself a complex one, and involves (1) the mechanism con- 

 trolling the rate and force of the heart beat and (2) the 

 mechanism controlling the caliber of the blood vessels. 

 When the cardio-regulatory and vasomotor nervous 

 mechanisms are intact, a fall in blood pressure is attended 

 by an increase in the rate of the heart beat; and, con- 

 versely, when the blood pressure tends to rise, the rate 

 of the heart decreases. When the extrinsic nerves to the 

 heart are cut, these changes in the pulse rate no longer 

 occur as an accompaniment to the changes in blood pres- 

 sure. The importance to the animal of these changes in 

 heart rate with changes in blood pressure is shown by 

 the fact that, rabbits and dogs whose extrinsic cardiac 

 nerves have been cut soon get out of breath on attempting 

 to run. 25 Through the combined agencies of the vaso- 

 motor and the cardio-regulatory nervous mechanisms, 

 the blood pressure in all mammals so far investigated 26 

 and in some birds, e. g., ducks and fowls, 27 is very much 

 the same, that is, about equal to a pressure of one hun- 

 dred and twenty-live millimeters of mercury. Changes 

 in the caliber of the blood vessels or in the rate of the 

 heart beat equalize the local changes of pressure due to 

 changes in muscular activity. Working glands or muscles 

 receive, as a rule, more blood than similar organs at rest. 

 This increase in blood-supply may be due in part to the 

 action of metabolites upon the walls of the blood vessels 

 of the active structure (Barcroft). 



The blood pressure in the homoiothermal animals, or 

 the blood pressure during the periods of activity of such 



25 See Guthrie and Pike, American Journal of Physiology, 1907, XVIII, 

 PI> 26 Porter and Richardson, American Journal of Physiology, 1908, XXIII, 

 P 2? Biddle and Matthews, American Journal of Physiology, 1907, XIX, 



