THE BLOODVESSELS 89 



centre appear to be reversed, and excitation of pressor fibres 

 causes a fall instead of a rise in pressure. If the centre be under 

 the influence of strychnine, stimulation of the depressor nerve, 

 instead of causing a fall in the general blood-pressure, produces 

 a rise. 



The vasomotor centres, both in the medulla and cord, are 

 extremely sensitive to the varying amounts of carbon dioxide 

 in the blood, and in this way they are believed to be capable 

 of a good deal of self-regulation, apart from the afferent impulses 

 which flow in to their aid. An increased venous condition of 

 blood leads to a constriction of the arterioles and a raising of 

 the blood-pressure. In asphyxia the arterioles remain con- 

 stricted under the influence of the now intensely venous blood 

 as it stimulates the vasomotor centre to unwonted activity, 

 and though the initially high blood-pressure subsequently falls 

 to zero, it does not do so because the arterioles have relaxed, but 

 because the heart has failed. 



It has been supposed that one cause of surgical shock is a defi- 

 ciency of carbon dioxide in the blood. The prevention of this 

 complication by avoiding a too free ventilation in the lungs, 

 and so maintaining a carbon dioxide blood-pressure, has been 

 recommended as a means of combating shock. The alarming 

 depression which after a prolonged operation not infrequently 

 follows the removal of the closed chloroform mask employed in 

 chloroforming horses is doubtless due to the fall of blood- 

 pressure caused by less C0 2 circulating in the vasomotor centre. 



Rhythmical Activity of the Vasomotor Centre. — Under certain 

 conditions, such as asphyxia and haemorrhage, the vasomotor 

 centre transmits to the vessels rhythmic constrictor impulses, 

 which result in the appearance, on a simultaneous record of 

 blood-pressure, of undulations, known as Traube-Hering curves 

 (Fig. 39). They can, of course, only be detected by taking a 

 tracing of the blood-pressure. The existence of these waves is 

 indicative of abnormal excitation of the vasomotor centre. 



The Dilator Nerves. — Hitherto we have mainly dealt with 

 the constrictor influence exercised over the bloodvessels, but the 

 nervous system likewise produces a dilator effect. In contrast 

 to the constrictor influence, £he dilator is not tonic in its action. 

 It might be supposed that a dilator effect would naturally follow 

 as the result of removing a constrictor influence from a vessel, 

 without the intervention of a separate or antagonistic nerve 

 supply ; and this is exactly what does happen in 'most cases. 

 But it is equally certain that special vaso-dilator nerves exist, of 

 which the chorda tympani is one of the best examples. This 

 nerve supplies the bloodvessels of the submaxillary gland with 

 dilator fibres ; if the nerve be cut no evident change in the blood- 



