THE CIRCULATION. 321 



sory and subsidiary centers in the spinal cord, and vasomotor nerves. 

 The nerves are divided into two classes, according as they increase or 

 diminish the caliber of the arterioles: those which increase the cali- 

 ber are vasodilators; those which diminish the same are known 

 as vasoconstrictors. All nerves that in any way influence vessel- 

 caliber are classed under the general head of vasomotor. 



How the Nerves End. The manner in which the nerves end in 

 the walls of the blood-vessels is an important subject. According 

 to the majority of histologists, they end in the circular muscle of 

 the arterioles. With the exception of the portal system, there has 

 not been established any direct proof of function of vasomotor nerves 

 in regard to the venous system. 



Stilling, in 1840, knew that the vascular nerves ran in the sym- 

 pathetic, and he named these nerves vasomotors. Claude Bernard, 

 in 1851, found that after section of the cervical sympathetic the 

 blood-vessels of the ear dilated and the ear became warmer. In 

 1853, Brown-Sequard discovered " that electrical irritation of the 

 cranial end of the sympathetic was followed by a contraction of the 

 blood-vessels, and that this contraction was followed by a cooling of 

 the ear. 



In 1858, Bernard found that when the chorda tympani was irri- 

 tated the blood-vessels, instead of being constricted, were dilated. 

 To such an extent did dilatation occur that the blood in the vein 

 acquired, instead of a blue color, a red color. The veins themselves 

 became swollen in size. 



These various observations tend to prove that there are two 

 kinds of vasomotor nerves : vasoconstrictors and vasodilators. 



Functions. Ordinarily the arterioles are in a state of tonicity 

 moderate contraction to maintain peripheral resistance; otherwise 

 the flow of blood through the capillaries would be intermittent 

 instead of continuous, as it normally is. It is when this peripheral 

 resistance is low that there appears a capillary and venous pulse. 



In hot weather the capillaries of the skin dilate ; in cold weather 

 they contract. 



Another very important function of the vasomotors is their 

 regulation of the amount of blood-supply to any part, organ, or gland 

 of the economy. That is, they govern the amount found within the 

 arterioles and capillaries of the tissues. 



The vasoconstrictor nerves arise from a center in the medulla 

 oblongata, pass down the lateral columns, and establish communica- 

 tion with minor vasomotor centers in the spinal cord, and then from 



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