252 THE CIRCULATION OF THE BLOOD 



The Relation of Vaso-constrictor and Vaso-dilator Activity. The 

 distribution of two sets of regulative fibers for the muscular walls of the 

 blood vessels, when considered in connection with the other factors of the 

 vascular apparatus, gives a wonderfully complete mechanism for the co- 

 ordination of the vascular supply with the activity of the different organs. 

 General and broadly distributed activity of the constrictors produces 

 increase of general blood pressure and of the dilators decrease of pressure, 

 but local activity of either set will produce a great reduction or increase of 

 blood in the local organ with little or no effect on the general pressure. 

 When a vaso-dilatation is produced locally in one organ and there is an 

 accompanying vaso-constriction in other regions, as usually happens, it is 

 evident that the result may be a flooding of the local region. This is 

 exactly the thing that is accomplished in the muscles in violent exercise, 

 in the glands during secretion, in the stomach during digestion. It is 

 this mechanism that is utilized to throw a large volume of blood to the 

 skin when the temperature of the body is above the average, or to blanch 

 the skin when the temperature is low. 



Normally certain regions of the body are associated in that when vaso- 

 dilatation occurs n one region, vaso-constriction occurs in the other. This is 

 particularly true with the skin or surface of the body and the viscera or deeper 

 organs. The same relation is said to exist between some of the visceral 

 organs. 



General Course of the Vaso-constrictor and Vaso-dilator Nerves. 

 The cell bodies forming the medullary vaso-motor center give off axones, 

 axis- cylinder processes, some of which go to the nuclei of origin of certain 

 cranial nerves, while others pass down the cord to end at different levels 

 in contact with certain cells, probably small cells in the anterior horn and 

 lateral part of the gray matter. These cord cells constitute the spinal centers. 

 The neuraxones of the spinal cells leave the cord in certain spinal nerves in 

 the anterior roots, pass by the white rami to the sympathetic ganglionic 

 chain, where they end in physiological connection with the ganglionic cell. 

 Axones from these latter cells pass by an uninterrupted course to their termina- 

 tions on the blood vessel walls. The vaso-constrictor fibers leave the central 

 nervous axis by the ventral roots of all the dorsal nerves and the first two 

 lumbar roots, a comparatively restricted region. The vaso-dilators have 

 the same origin with two exceptions, viz., the vaso-dilators of the salivary 

 glands found in the seventh and ninth cranial nerves, and the nervi erigentes, 

 which arise in the roots of the second and third sacrals. 



The nerves to the viscera pass direct to their blood vessels, but the vas- 

 cular nerves for the skin, muscles, limbs, etc., rejoin the main divisions of the 

 spinal nerves through the gray rami, see figures 417 and 418, and pass to the 

 blood vessels along with the general nerves of the organ or organs. 



