THE CIRCULATION OF THE BLOOD. 367 



beyond the natural condition by the side pressure of the blood flowing 

 through them, at the same time the muscle fibers are in a state of continuous 

 contraction, more or less pronounced, and give to the arteries a certain 

 average caliber which permits a definite volume of blood to flow through 

 them in a given unit of time. To this condition of the arterial wall the 

 term tonus is applied. 



The cause of this tonic contraction is not definitely known. It has 

 been attributed to the action of local nerve-ganglia, to the pressure of blood 

 from within, to the influence of organic substances in the blood, the prod- 

 ucts of gland activity: e.g., adrenalin or epinephrin. 



This tonic contraction of the vascular muscle is subject to increase or 

 decrease, augmentation or inhibition, in accordance with the action of various 

 agents. An augmentation of the contraction will result in a decrease of 

 the caliber and a reduction in the outflow of blood. An inhibition of the 

 contraction or relaxation will result in an increase both of the caliber and 

 outflow of blood. The small arteries thus determine the volume of blood 

 passing to any given area or organ in accordance with its functional 

 activities. 



The Vaso-motor Nerves. The activities of the vascular muscle are 

 regulated by the central nerve system through the intermediation of nerve- 

 fibers, termed vaso-motor nerves. Of these there are two kinds, one which 

 increases or augments the contraction, the vase-constrictors or vaso-aug- 

 mentors; and another which decreases or inhibits the contraction, the vaso- 

 dilatators or vaso-inhibitors. 



The vaso-motor nerves of both classes, unlike the ordinary motor nerves, 

 do not pass directly to the muscle-fiber, but indirectly by way of the ganglia 

 of the sympathetic nerve system. In these ganglia the vaso-motor nerves, 

 which come from the central nerve system, terminate, breaking up into 

 tufts, which arborize around the nerve-celts. From the cells of these gang- 

 lia new nerve-fibers arise which then pass without interruption to their 

 final destination. 



The nerve-fibers which emerge from the central nerve system are ex- 

 tremely fine in caliber and medullated; those which emerge from the sym- 

 pathetic ganglia are equally fine, but non-medullated. The former are termed 

 pre-ganglionic or autonomic, the latter post-ganglionic or sympathetic fibers. 

 The ganglion in which the pre-ganglionic fibers end is not necessarily found in 

 the pre- vertebral or lateral chain; it may be found in the collateral or even in 

 the peripheral group of ganglia. (See sympathetic or autonomic nerve system.} 



The Vaso-constrictor Nerves. The vaso-constrictor nerves take their 

 origin from nerve-cells located in the anterior horns and lateral gray matter 

 of the spinal cord. They emerge from the cord in company with the fibers 

 that compose the ventral roots of the spinal nerves from the second or third 

 lumbar nerves inclusive. A short distance from the cord they leave the ventral 

 root as the white rami communicantes and enter the pre-vertebral or lateral 

 sympathetic ganglia. From the results of many observations and experi- 

 ments it is probable that the great majority of the vaso-constrictor nerves 

 terminate in these ganglia; that is to say, it is here that the pre-ganglionic 

 fibers arborize around the contained nerve-cells. From the nerve-cells 

 new fibers arise, the post-ganglionic, which pass to the blood-vessels of the 



