RESPIRATORY UNDULATIONS. 1 25 



the arteries to contract, and thus the blood-pressure is raised. The variations in 

 the pressure which depend upon a varying activity of the vaso-niotor centre are 

 known as the "curves of Traube and Hering." Fig. 107 shows the carotid blood- 

 pressure tracing of a dog. In this curve, when inspiration begins (I) the blood- 

 pressure is still falling slightly, but gradually rises until it reaches its maximum 

 shortly after the beginning of expiration (E). [The maxima and minima of the 

 respiratory and blood-pressure 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 (fig. 107), while in a rabbit 

 this difference of the pulse-rate is but slightly marked (fig. 106). The smaller 

 number of pulse-beats during the descent, i.e., during the greater part of expiration 



Fig. ]07. 

 Carotid blood-pressure tracing of dog ; vagi riot divided ; I = inspiration, E = expiration. 



{Stirling). 



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-motor centre in the medulla oblongata by the impure blood. This causes 

 contraction of the small arteries throughout the body, which retards the outflow 

 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 on the heart, or on any local peripheral nervous mechanism, or on the 



