170 RESPIRATORY UNDULATIONS IN THE BLOOD-PRESSURE CURVE. 



mences i.e., during the last part of the act of inspiration. This 

 is due to the contraction of the arteries, caused by impulses sent 

 from the vaso-motor centre. It is also aided by the circum- 

 stance that during inspiration there is an increased inflow of 

 venous blood to the heart, so that when it contracts, more blood is 

 forced into the arteries. [The maxima and minima of the two curves 

 do not coincide exactly, but in addition the number of pulse-beats is 

 greater in the ascent than in the descent. This is well-marked 

 in a blood-pressure tracing from a dog's carotid, while in a 

 rabbit this difference of the pulse-rate is but slightly marked. The 

 smaller number of pulse-beats during the descent i.e., during the 

 greater part of expiration is due to the activity of the cardio- 

 inhibitory centre in the medulla oblongata. This is proved by the fact, 

 that section of both vagi in the dog causes the difference of pulse-rate 

 to disappear, while other conditions remain the same as before, except 

 that the heart beats more rapidly. It would seem that during the ascent, 

 the cardio-inhibitory centre is comparatively inactive. It is clear, 

 therefore, that the respiratory and cardio-inhibitory centres in the medulla 

 oblongata act to a certain extent in unison, so that it is reasonable to 

 suppose that other centres situated in close proximity to these may 

 also act in unison with them, or, as it were, " in sympathy." As 

 already stated, the vaso-motor centre is also in action during a particular 

 part of the time.] 



[If a dog be curarised and artificial respiration established, the 

 respiratory undulations still occur, although in a modified form. In 

 artificial respiration, the mechanical conditions, as regards the intra- 

 thoracic pressure, are exactly the reverse of those which obtain during 

 ordinary respiration. Air is forced into the chest during artificial 

 respiration, so that the pressure within the chest is increased during 

 inspiration, while in ordinary inspiration the pressure is diminished. 

 Thus, the same mechanical explanation will not suffice for both cases.] 



If the artificial respiration be suddenly interrupted in a curarised 

 animal, the blood-pressure rises steadily and rapidly. This rise is due 

 to the stimulation of the vaso-rnotor centre in the medulla oblongata by 

 the impure blood. This causes contraction of the small arteries 

 throughout the body, which retards the out-flow from the large 

 arteries, and thus the pressure within them is raised. [Stated 

 broadly, the arterial pressure depends on the central organ the 

 heart, and on the condition of the peripheral organs the small 

 arteries. Both are influenced by the nervous system. If the action of 

 the vaso-motor centre be eliminated by dividing the spinal cord in the 

 cervical region, arrest of the respiration causes a very slight rise of the 

 blood-pressure ; hence, it is evident that venous blood acts but slightly 



