CHAP, iv.] THE VASCULAR MECHANISM. 207 



central nervous system, the afferent impulses being for the most part 

 carried by ordinary sensory nerves, while the efferent impulses pass 

 along special vaso-motor fibres, which, though the centre of the 

 reflex action lies in the cerebro-spinal axis, have a great tendency 

 to run in sympathetic tracts. 



The afferent impulses of course need not start from the peri- 

 pheral nerve-endings. They may for instance arise in the brain. 

 Thus, as we have seen, an emotion originating in the cerebrum 

 may by vaso-motor action give rise either to blushing or to pallor. 

 Nay more, changes may be induced in the central nervous system 

 itself without the need of any impulses reaching it from without. 

 When we come to discuss the relations of respiration to the 

 circulation, we shall see reason to think that the vaso-motor action 

 of the central nervous system may be directly affected by the 

 condition of the blood passing through it, so that if the quantity of 

 oxygen in the blood be reduced, a general arterial constriction 

 takes place, and a rise of blood-pressure follows; while with a 

 return of oxygen to the blood, the vessels dilate and pressure falls. 

 And it is more than probable that many substances introduced 

 into the blood, or arising in the blood from natural or morbid 

 changes, may affect blood-pressure by acting directly on the 

 centres in the central nervous system. They may also however act 

 on the peripheral structures. We shall return to these phenomena 

 later on. 



In many ways then, and to a varying degree and extent, the 

 central nervous system can bring about arterial constriction or 

 dilation, general or local. We have now to study the question, 

 What is more exactly the nature of the nervous influences 

 which lead to constriction and dilation respectively? How do 

 those which cause constriction differ from those which cause 

 dilation ? 



In the fundamental experiment of the cervical sympathetic, 

 when arterial dilation has followed upon section of the nerve, if the 

 peripheral stump of the divided nerve be stimulated, the dilation 

 gives place to constriction, the blush is replaced by pallor. If the 

 stimulus be very strong the constriction is greater than normal, but 

 by carefully adjusting the strength of the stimulus, the circulation 

 may be brought to quite a normal condition, the ' loss of tone ' 

 consequent on the severance of the vaso-motor fibres from the 

 central nervous system may be replaced, and not more than 

 replaced, by an artificial tone generated by the action of the 

 stimulus on the sympathetic nerve. The most natural interpreta- 

 tion therefore of the vaso-motor action in this case is to suppose 

 that the normal tone of the arteries of the face is maintained by 

 'tonic' constructive impulses of a certain intensity which pass 

 from the central nervous system along the sympathetic, and that 

 the dilation of the same arteries is due simply to a diminution or 

 absence of these constrictive impulses, an increased constriction or 



