THE CIRCULATION OF THE BLOOD AND LYMPH 171 



nic area ; but 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. 72 and 73). It has been shown that in both 

 cases vaso-constriction of 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 



FIG. 71. PRESSOR EFFECT OF STIMULATION OF 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.) 



always, whether the shocks follow each other rapidly or slowly, 

 a rise of general blood-pressure, mechanical stimulation of a 

 muscle, as by kneading or massage, causes a fall. The condi- 

 tion 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 ex- 

 citation of which is in all circumstances followed by a general 



