THE CIRCULATION OF THE BLOOD 377 



traction, thus giving 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 de- 

 crease, to 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 vase-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-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 thoracic 

 to the second or third lumbar nerves inclusive. A short distance from the 

 cord they leave the ventral roots as the white rami communicantes and enter 

 for the most part the vertebral or lateral sympathetic ganglia. From the 

 results of many observations and experiments 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 (i) the body walls; (2) the fore limbs; (3) 

 the head, neck and face; (4) the hind limbs; and (5) the abdominal viscera, 

 i. The vaso-constrictors for the blood-vessels of the walls of the trunk of 

 the body emerge from the spinal cord in the ventral roots of the spinal 

 nerves from the second thoracic to the third lumbar nerves inclusive. 

 After a short course they leave the ventral roots and pass by way of the 

 white rami into the corresponding thoracic and lumbar ganglia around 

 the nerve-cells of which they arborize. From these ganglia new nerve- 

 fibers arise which pass back by way of the gray rami, into the thoracic 

 and lumbar nerves and in association with them pass directly t 

 blood-vessels of the skin. (Fig. 178 A.} 



2 The vaso-constrictors for the blood-vessels of the fore limbs emerge from 

 the spinal cord in the ventral roots of the fourth to the tenth thoracic 

 nerves inclusive. After a short course they leave the ventral roots pass 

 into the white rami, thence into the sympathetic chain after wh 

 take an upward direction and terminate around the cells of the ga 

 stellatum From this ganglion the new nerve-fibers enter by way of 

 the gray rami, the trunks of the cervical nerves which unite to form t] 



