RESPIRATORY WAVE IN BLOOD-PRESSURE CURVE. 301 



We may conclude, then, that a sympathy in action can be dis- 

 tinctly recognized in the working of the respiratory and cardiac 

 nerve centres. 



Since the undulations known asTraube's curves occur in cura- 

 rized animals when no respiratory movements are performed, it has 

 been proposed to explain the respiratory undulations in the same 

 way, namely, by referring them to a stimulation of the vasomoter 

 centre by impure blood, which by rhythmical impulses increasing 

 the contraction of the arterioles causes a rhythmical variation in 

 the blood pressure. This explanation seems to be rendered unsat- 

 isfactory by the fact that the respiratory undulations go on even 

 when the arterioles are cut off from their chief nerve centres by 

 sections of the spinal cord. So that if these undulations are to 

 be referred to nerve mechanism, we are ignorant of the course the 

 nerve impulses take, for any rhythmical sympathy existing be- 

 tween the respiratory and vasomotor nerve centres in the medulla 

 cannot have any influence when the cord is cut. 



The blood pressure in the capillaries cannot be directly measured ; 

 it is difficult to estimate, and very variable. The slightest change 

 of pressure in the corresponding veins or arteries causes the press- 

 ure in the capillaries to rise or fall. Thus variations in pressure 

 are constantly occurring in the capillaries, which cause an alter- 

 ation in the rate of flow, or even a retrograde stream in some 

 parts of the network. 



The regulation of the blood supply, and, therefore, of the press- 

 ure in the capillaries, is under the control of the small arterioles 

 which supply them ; a slight relaxation of the muscle of the arteri- 

 oles causes great increase in the amount of blood flowing through 

 the capillaries, as can readily be seen with the microscope. 



The blood pressure in the veins must be less than that in the 

 capillaries, and, as has been said, must diminish as the heart is 

 approached, where in the great veins (superior cava) the pressure 

 is said to be rather below that of the atmosphere ( 3 to 5 

 mm., mercury). During inspiration the minus pressure may be- 

 come much less, whilst, on the other hand, it is only by very 

 forced expiration that it ever becomes equal to or at all above 

 that of the atmosphere. 



