VASOCONSTRICTOR AND VASO-DILATOR ACTIVITY 219 



be applied to the action of the depressor nerve described above, page 217. 

 On the whole, however, while we cannot directly and unquestionably prove 

 the fact, yet it is probable that each of the above examples may be accepted 

 as examples of reflex vaso-dilatation by direct action on a vaso-dilator center 

 or centers in the cord. 



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 coordi- 

 nation 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, 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-constric- 

 tion 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 in 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 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 ganglion chain, 

 where they end in physiological connection with the ganglionic cells. Axones 

 from these latter cells pass by an uninterrupted course to their terminations 

 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 vascular nerves for the skin, 



