THE BLOODVESSELS 91 



travel in a direction opposite to the normal. Such, however, 

 is the only explanation at present offered — viz., that the fibres 

 act in a double capacity, conveying to the cord sensory impulses 

 from the limbs, and by the same fibres conveying efferent or out- 

 going impulses from the cord to the vessels, resulting in their 

 dilatation. 



The dilator nerves in their distribution — and we here refer to 

 those whose existence is undoubted — behave very differently 

 from the constrictor fibres. They leave the brain or cord by 

 any cerebro-spinal nerve, and may or may not pass into a sympa- 

 thetic ganglion before distribution. In contrast to the constric- 

 tors, they pass direct to their destination instead of taking a 

 roundabout course, and they do not lose their medulla until 

 near their termination. There is no positively known centre in 

 the medulla governing the vaso-dilator fibres. There is, however, 

 undoubted evidence of such a centre in the cord, for erection 

 of the penis will still occur as a normal reflex after the cord has 

 been divided above the lumbar region. 



All the ' depressor ' influences exercised on the circulation are 

 not produced through inhibition (withholding) of constrictor 

 impulses. Some must occur through stimulation of dilator 

 nerves, but of this very little is positively known. 



It is evident from what has been said that the knowledge of 

 dilator nerves for the body tissues generally is still in a very 

 uncertain condition. 



We have seen the general effects produced by vaso-constrictor 

 and vaso-dilator nerves, and indicated some of the gaps which 

 exist in our knowledge. The broader features appear to admit 

 of no doubt. The size of the vessels has to be automatically 

 regulated according to the demand made for blood, so that a 

 double set of fibres is provided, one to constrict and one to 

 dilate them. A general contracted condition of the vessels must 

 be maintained if the circulation is to continue ; consequently, 

 constrictor impulses appear always more in evidence than 

 dilator. These constrictor effects are assisted by the contractile 

 reaction of the arterial wall, the result of the peculiar property 

 of smooth muscle, and this contractile reaction is brought into 

 play, even apart from nervous impulses, by the thrust given to 

 the blood at each contraction of the heart. Substances circu- 

 lating in the blood, the secretion of such glands as the adrenals, 

 of which we shall learn more presently, act on the nerve plexus 

 in the walls of the bloodvessels, constricting them and raising the 

 pressure. The heart centre in the medulla is being constantly 

 informed of what is occurring in the two vast systems — skin and 

 splanchnic — which between them regulate the blood-pressure of 

 the body. Equally, impulses are constantly streaming out from 



