THE NERVOUS REGULATION OF THE BLOODVESSELS 183 



surgical shock (Henderson). In addition to the direct influence of 

 carbon dioxide, and possibly of other substances, the arrival of 

 afferent impulses at the centre seems to play a part in maintaining 

 that continual discharge of efferent impulses along the vaso-motor 

 nerves which constitutes 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 s 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 that 

 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 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 

 cord, and the other in a sympathetic ganglion. Less is known of the 

 elements which constitute the bulbar centre and of their connections. 

 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, irom the bulb to the vessels of the leg, would have 

 to traverse three neurons (p. 823). 



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 ana- 

 tomical 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 im- 

 portant 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 dila- 

 tation of the arterioles throughout the body. That the depressor 

 action involves excitation of vaso-dilators follows from the fact thai 

 vaso-dilatation occurs in the limbs on stimulation of the depressoi 

 after their vaso-constrictor nerves have been cut. Stimulation of 



