THE NERVOUS REGULATION OF THE BLOODVESSELS i87 



superficial parts, as the lips, may be seen to be flushed with blood. 

 In asphyxia, when the vaso-motor centres are directly stimulated by 

 the venous blood, this apparent antagonism is still better marked : the 

 cutaneous vessels are widely dilated and engorged, the face is livid, 

 but the abdominal organs are pale and bloodless (Heidenhain) . The 

 blood-pressure rises rapidly, reaches a maximum, and then gradually 

 falls as the vaso-motor centre becomes paralyzed (Figs. 84 and 85). It 

 has been shown that in both cases vaso-constriction ot the skin is really 

 produced as well as vaso-constriction of the internal organs, but the 

 increased blood-pressure mechanically overcomes the constriction of 

 the cutaneous vessels. 



The kind of stimulus seems to have something to do with the 

 direction of the reflex vaso-motor change. For while electrical 

 stimulation of every muscular nerve, even of the very finest twigs 

 that can be isolated and laid on electrodes, provokes always, whether 

 the shocks follow each other rapidly or slowly, a rise of general 



Fig. 83. Pressor Effect of Stimulation cf Central End of Vagus in a Cat during 

 Resuscitation after Cerebral Anaemia. The depressions in the signal line ABC 

 indicate the duration of three successive excitations of equal strength, sixty-five, 

 seventy-three, and seventy-nine minutes respectively after restoration of the 

 circulation. The pressor effect increases as resuscitation proceeds. Later on 

 the original depressor effect was again obtained. The upper tracing is that of 

 the artificial respiration. (Two-thirds original size.) 



blood-pressure, mechanical stimulation of a muscle, as by kneading 

 or massage, causes a fall. The condition of the afferent fibres also 

 exerts an influence. For example, excitation of the central end of a 

 sciatic nerve that has been cooled is followed by vaso-dilatation 

 and fall of pressure, the opposite of the ordinary result. These and 

 similar facts have led to the idea that most afferent nerves contain 

 two kinds of fibres, whose stimulation can affect the activity of the 

 vaso-motor centres ' reflex vaso-constrictor,' or ' pressor ' fibres, 

 and ' reflex vaso-dilator/ or ' depressor ' fibres. The branch of the 

 vagus, however, to which the name ' depressor * has been specially 

 given is usually described as the only peripheral nerve'the excitation 

 of which is in all circumstances followed by a general diminution of 

 arterial pressure. But this is not strictly correct, for at an early 

 period in the resuscitation of the brain after anaemia excitation of 



