24 



COLLATERAL VESSELS. 



actions of muscular fibres are controlled and regulated by the action of the vaso- 

 motor nerves (S 371). 



(e) Collateral Vessels. The arterial pressure within a given area of the vascular 

 system must rise or fall according as the neighbouring areas are diminished, whether 

 by the application of pressure, or a ligature, or are rendered impervious, or as these 

 areas dilate. The application of cold or warmth to limited areas of the body in- 

 creasing or diminishing the atmospheric pressure on a part the paralysis or stimu- 

 lation of certain vaso-motor areas ($ 371), all produce remarkable variations in the 

 blood-pressure. [The effect of dilatation of a large vascular area on the arterial 

 pressure is well shown by what happens when the blood-vessels of the abdomen are 

 dilated. Divide both vagi in the neck of a rabbit and stimulate the central end of 

 the superior cardiac or depressor nerve ; after a few seconds, the blood-vessels of 

 the abdomen dilate, and gradually there is a steady fall of the blood-pressure in the 

 systemic arteries. Fig. 106 is a blood-pressure tracing showing the height of the 

 blood-pressure before stimulation, a. The stimulation was continued from a to b, 

 and after a certain latent period there is a steady fall of the blood-pressure.] 



(/) Respiratory Undulations. The arterial pressure also undergoes regular 

 variations or undulations owing to the respiratory movements. These undulations 



Fig. 106. 

 Kymographic tracing showing the effect on the blood -pressure of stimulation of the central end 

 of the depressor nerve in the rabbit after section of both vagi in the neck. Stimulation 

 began at a and ended at b ; o-x, the abscissa. 



are called respiratory undulations (figs. 102 and 107.) Stated broadly, during 

 every strong inspiration the blood-pressure rises, and during expiration it falls 

 ( 74). This is not quite correct. These undulations may be explained by the 

 fact that, with every expiration, the blood in the aorta is subjected to an increase 

 of pressure through the compressed air in the chest ; with every inspiration, on the 

 other hand, it is diminished owing to the rarefaction of the air in the lungs acting 

 upon the aorta. Besides, the inspiratory movements of the chest aspirate blood 

 from the vena? cava? towards the heart, while expiration retards it, and thus 

 influences the blood-pressure. The undulations are most marked in the arteries 

 lying nearest to the heart. The respiratory undulations are due in part to a 

 stimulation or condition of excitement of the vaso-motor centre, which runs parallel 

 with the respiratory movements. This stimulation of the vaso-motor centre causes 



