KESPIEATIOtf. 607 



of the veins, the effect of inspiration in producing expansion of the blood- 

 vessels is less marked on the arteries than on the veins, and, as a conse- 

 quence, the reduction of blood pressure from arterial expansion is less 

 marked than the increase of blood pressure from venous expansion. 

 During expiration, on the other hand, increase of pressure without the 

 aortic arch acts, of course, in a similar manner to reduction of the calibre 

 of the aorta, and so would lead to an increase in the blood pressure. This 

 reduction is, however, likewise less than the compensating reduction in 

 the volume of the veins ; so that although expiration would therefore tend 

 to increase arterial pressure, it is more than compensated for by the 

 reduced amount of blood brought to the heart through the veins. 



If, however, a simultaneous tracing be made of the blood pressure 

 and of the respiratory movements, it will be seen that the increase of 

 blood pressure does not exactly coincide with the commencement of 

 inspiration, nor is the fall of blood pressure synchronous exactly with 

 expiration ; but it will be found that at the commencement of inspira- 

 tion blood pressure is falling, while it begins to rise before inspiration 

 is completed, and does not attain its maximum until after expiration has 

 commenced. Then, before expiration is completed, the fall again com- 

 mences, and continues during the first part of the succeeding inspiration. 

 These facts show that, in addition to the mere mechanical changes in the 

 blood supply produced by the varying pressure in inspiration and expira- 

 tion, some other causes are at work. 



The respiratory undulations in the blood pressure are clue, in part, 

 to varying degrees of stimulation of the vaso-motor centre. In other 

 words, the degree of tonicity of the blood-vessel walls is subject to 

 rhythmical variations, these variations depending upon coincident 

 changes in the degree of stimulation of the vaso-motor centre, to which 

 are added the changes produced in a purely mechanical way by altera- 

 tions in the intra-thoracic pressure. 



If the pulsations be counted in the ascending and descending phases 

 of the blood pressure, it will be noticed that the pulse beats faster dur- 

 ing increase of pressure than where it is decreasing. This variation in 

 the pulse-rate is especially marked in the dog, and is due to changes in 

 the activity of the eardio-inhibitory centre in the medulla oblongata, as 

 is proven by the fact that section of both vagi causes the difference in 

 pulsation to disappear. 



It is therefore evident that the vaso-motor, the respiratory, and the 

 eardio-inhibitory centres to a certain extent act in unison. 



The state of affairs is different in cases where artificial respiration is 

 carried on, as in curarized dogs, when the mechanical causes at work 

 must necessarily be reversed. If a dog be curarized and subjected to 

 artificial respiration, and tracings of the intra-thoracic pressure and of 



