THE CIRCULATION OF THE BLOOD 



693 



pressure is not entirely due to the cardiac inhibition, was shown by the fact 

 that it was still present when the vagus nerves were cut, and no change in the 

 heart beat occurred. Fig. 127 (page 423) is a typical form of depressor curve 

 in the rabbit with vagi cut. The fall of blood pressure is, in fact, produced by 

 general vaso-dilatation in all organs of the body which are supplied by vasomotor 

 nerves. Fig. 238 shows that it occurs in the intestine, and Fig. 120 (page 412) 

 that it occurs in the leg. 



The peripheral receptor ends of the depressor nerve are situated not only 

 in the heart but in the arch of the aorta, and its function appears to be to protect 



FIG. 241. EXCITATION OF VASO-UILATORS IN DEPRESSOR REFLEX. 



Arterial pressure in cat, upper curve. Zero, 33 mm. below time signal. 



Upper signal line, drops of blood from vein of submaxillary gland. 



Middle signal, stimulation of central end of vagus. 



Lower signal, time in two-second intervals. 



Cervical sympathetic cut, so that the gland was supplied with vaso-dilator fibres in the 



chorda tympani nerve, and these were the only vasomotor nerves present. 

 The first part of the fall of pressure is accompanied by vascular dilatation in the gland, 



shown by the more rapid succession of drops. The later continued fall, after 



ceasing the stimulation, is, no doubt, due to failure of the heart from insufficient 



blood supply, and not to peripheral dilatation. 



(Bayliss, 1908, 3, Fig. 1.) 



the heart from too great a rise of blood pressure. We see in Fig. 106 (page 386) 

 how it is excited at each heart beat, but it has been found difficult to show 

 experimentally that a rise of aortic pressure, produced otherwise than by each 

 heart beat, stimulates it. In most mammals the depressor fibres are contained 

 in the trunk of the vagus, and it is not always possible to obtain their effect apart 

 from the opposite pressor effect of the ordinary sensory fibres in the vagus trunk. 

 In the cat the vagus acts as a depressor nerve only. 



Fig. 238 gives instances of a reflex rise and a reflex fall of arterial pressure, 

 and the plethysmograph curve of the intestine shows that the rise is produced 

 by a peripheral vaso-constriction and the fall by dilatation. 



Now we have seen that there are both constrictor and dilator fibres leaving 

 the central nervous system, so that there must be centres from which they arise. 



