160 A MANUAL OF PHYSIOLOGY 



be deduced from the physiological facts we have been reciting. It 

 has been surmised that certain cells of small size scattered up and 

 down the cord in the anterior horn and intermedio-lateral tract, and 

 cropping out also in the bulb, are vaso-motor cells. It must be 

 assumed that their axis-cylinder processes are connected with the 

 vaso-motor fibres which we have already discovered emerging from 

 the brain in certain cranial nerves and from the cord in the anterior 

 spinal roots. And, indeed, there is reason to believe that, in the case 

 of the spinal vaso-motor cells at any rate, the connection is made 

 without the intervention of any other nerve-cells, and that the axis- 

 cylinders of these vaso-motor fibres are the axis-cylinder processes of 

 the vaso-motor cells. So that the simplest efferent path along which 

 vaso-motor impulses can pass may be considered as built up of two 

 neurons, one with its cell-body in the central nervous system, and the 

 other in a sympathetic ganglion. But since it would appear that the 

 spinal vaso-motor centres are under the control of the chief centre 

 in the bulb, it is necessary to suppose that the axis-cylinder processes 

 of some of the cells of the bulbar centre come into relation with the 

 spinal vaso-motor cells, and that impulses passing, let us say, from 

 the bulb to the vessels of the leg, would have to traverse three 

 neurons (see Chap. XII.). 



Vaso-motor Reflexes. We have already seen that the 

 cardiac centres are constantly influenced by afferent im- 

 pulses, and that in the direction either of augmentation or 

 inhibition. The vaso-motor centre in the bulb is equally 

 sensitive to such impulses. They reach it for the most part 

 along the same nerves, and by increasing or diminishing its 

 tone cause sometimes constriction and sometimes dilatation 

 of the vessels, the result depending partly upon the anato- 

 mical connection of the afferent fibres, but apparently in 

 part also upon the state of the centre. 



Of the afferent nerves that cause vaso-dilatation, the most 

 important is the depressor, whose reflex inhibitory action on 

 the heart has been already described. The fall in the 

 arterial pressure is due chiefly, not to the inhibition of the 

 heart, but to the inhibition of the portion of .he vaso-motor 

 centre that presides over the great area ruled by the 

 splanchnic nerves, and the consequent dilatation of the 

 vessels of the abdominal viscera. For if these nerves have 

 been previously cut, stimulation of the depressor is ineffective, 

 while it produces its usual result after section of the vagi. 

 It has been suggested that the function of the depressor is 

 to act as an automatic check upon the blood-pressure in the 



