THE CIRCULATION OF THE BLOOD AND LYMPH 169 



of efferent impulses along the vaso-motor nerves which consti- 

 tutes its tone. In this regard, the vaso-motor centre occupies 

 an intermediate position between the respiratory centre, the 

 most purely automatic, and the cardio-inhibitory centre, the 

 most purely reflex of the three great bulbar mechanisms. 



Of the anatomical relations of the nerve-cells that make up the 

 bulbar and spinal vaso-motor centres, little more is known than may 

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

 has been surmised on histological grounds that certain cells of small 

 size scattered up and down the thoracic and upper lumbar regions 

 of the cord in the lateral horn (intermedio-lateral tract), and perhaps 

 cropping out also in the bulb, are vaso-motor cells. There is good 

 evidence thaf the pre-ganglionic sympathetic fibres, including the 

 vaso-motor fibres, which we have already discovered emerging from 

 the cord in the spinal roots, are connected with these cells. And, 

 indeed, there is reason to believe that the connection is made with- 

 out 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 cord, and the other in a 

 sympathetic ganglion. Less is known of the elements which con- 

 stitute the bulbar centre and of their connections. But since it 

 would appear that the spinal vaso-motor centres are under the con- 

 trol 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 im- 

 pulses 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 impulses, 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 anatomical 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 

 inhibition of the vaso-constrictor tone of the bulbar vaso-motor 

 centre, combined with stimulation of vaso-dilator nerves, and 

 consequent general dilatation of the arterioles throughout the 

 body. That the depressor action involves excitation of vaso- 

 dilators follows from the fact that vaso-dilatation occurs in the 

 limbs on stimulation of the depressor after their vaso-constrictor 

 nerves have been cut. Stimulation of the depressor produces its 

 usual result after section of the vagi. It has been suggested that 



