THE ARTERIES. 465 



influence, however slight, to which the body may he exposed. 

 Further, the cardiac centre is affected by its blood-supply, 

 including both the quality and pressure of the blood within it. 



Amongst the afferent impressions reaching the cardiac 

 centre, those from the heart itself travel through the vagus. 

 These are partly impressions of common sensibility, which 

 pass through the medulla into the convolutions ; and although 

 normally too feeble to be perceived, may, if powerful, give 

 rise to sensations of pain, distress, weight, and palpitation, re- 

 ferred to the praecordium. 



The arteries are active, irritable muscular tubes, whose 

 calibre can be modified by a variety of influences. A local 

 nervous mechanism guides the vasor muscles ; vaso-motor and 

 vaso-dilator nerves pass between the local mechanism and the 

 central nervous system ; and there is a great central point in 

 the medulla oblongata, called the vaso-motor centre, as well 

 as other lower centres in the cord and brain, which collect im- 

 pressions from every part of the body, and reflect them 

 through the vaso-motor or vaso-dilator nerves, as the case 

 may be, to the vessels. The muscular coat of the arte- 

 ries, being constantly exercised to a degree, gives so-called 

 " tone " to the vessels, which is one of the elements of that 

 cardinal factor of the circulation, the peripheral resistance. The 

 more active the vaso-motor nerves or centres, the greater the 

 resistance and the higher the blood pressure ; the more active 

 the dilator, the lower the pressure ; and the influence of each 

 upon the heart respectively corresponds. Particular vascular 

 areas, e.g. those of the skin and mesentery, may also be 

 dilated or constricted independently of others. Manifestly 

 local dilatation will admit more blood to the part, and so lower 

 the general arterial pressure ; local constriction will increase 

 the local resistance, and so raise the general pressure. Amongst 

 the impressions which influence the vaso-motor centre are 

 mental states, visceral conditions, surface temperature and 

 sensations of all kinds. It is also stimulated by deficiency 

 of blood within itself, and by poverty of the blood in oxy- 

 gen, and drugs act directly upon it as we shall presently see. 



The afferent impressions which reach the vaso-motor centre 

 from the heart are so important to the therapeutist that they 

 demand special mention. When impressions originating in over- 

 distension, distress, or failure of the heart, reach the cardiac 

 centre through the vagus, they are transferred to the vasor 

 centre, whence they are reflected to the vessels through the 

 dilator nerves. The vessels are thus relaxed; the arterial 

 pressure, which the ventricle has to overcome, falls ; the heart 

 empties itself more readily, and is relieved. This arrangement 

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