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VETERINARY PHYSIOLOGY 



by the respirations the rise in pressure in great measure 

 corresponding to the phase of inspiration, the fall in pressure 

 to the phase of expiration. This statement is not quite 

 accurate, as will be seen when considering the influence of 

 respiration on circulation (see page 311). These variations 

 are easily seen in a tracing of the arterial pressure taken with 

 the mercurial manometer (fig. 127, A). 



A pulse synchronous with the respirations may also be 

 observed in the great veins at the root of the neck and in 

 the venous sinuses of the cranium when it is opened. With 



FIG. 126. Tracing of the arterial Llood pressure to show large respiratory varia- 

 tions, aud small variations due to heart heats upon these, and the sudden 

 fall iu the pressure produced by stimulating the inferior cardiac branch of 

 the vagus nerve. 



each inspiration they tend to collapse, with each expiration 

 they again expand. The reason for this is that during inspira- 

 tion the pressure inside the thorax becomes low and hence 

 blood is sucked from the veins into the heart, while during 

 expiration the intra-thoracic pressure becomes higher and thus 

 the entrance of blood into the heart is opposed. 



3. Mean Blood Pressure 



I. PRESSURE IN THE ARTEKIES 



A. Methods 



The first investigation of the pressure in the blood vessels 

 was made by the Rev. Stephen Hales in 1733. He fixed a 





