CH. XXI.] THE VASO-MOTOR NERVOUS SYSTEM 297 



tracting there is a slow rise due to the fact that the blood cannot get 

 into the ventricle, and so distends the auricle ; a second short, sharp 

 elevation of pressure is produced by the auricular systole. Altera- 

 tions of venous pressure are also produced in the great veins by the 

 respiratory movements, the pressure sinking during inspiration, and 

 rising during expiration. 



The Vaso-Mofor Nervous System. 



The vaso-motor nervous system consists of the vaso-motor centre 

 situated in the bulb, of certain subsidiary vaso-motor centres in the 

 spinal cord, and of vaso-motor nerves, which are of two kinds (a) 

 those the stimulation of which causes constriction of the vessels ; 

 these are called vaso-constrictor nerves ; (&) those the stimulation of 

 which causes dilatation of the vessels ; these are called vaso-dilator 

 nerves. 



The following names are associated with the history of the subject. 

 The muscular structure of arteries was first described by Henle in 

 1841 ; in 1852 Brown Sequard made a study of the vaso-constrictor, 

 or, as he termed them, tonic nerves. The vaso-motor centre was dis- 

 covered by Schiff (1855), and more accurately localised by Ludwig 

 (1871). The dilator nerves were also discovered by Schiff; at first 

 they were termed paretic nerves. Other names which must be 

 mentioned in connection with the subject are those of Claude Bernard, 

 Heidenhain, and, in more recent years, G-askell, Langley, and Eamon 

 y Cajal. 



The nerves supply the muscular tissue in the walls of the blood- 

 vessels and regulate their calibre, but exert their most important 

 action in the vessels which contain relatively the greatest amount of 

 muscular tissue, namely, the small arteries or arterioles. 



Under ordinary circumstances, the arterioles are maintained in 

 a state of moderate or tonic contraction, and this constitutes the 

 peripheral resistance, the use of which is to keep up the arterial 

 pressure, which must be high enough to force the blood through the 

 capillaries and veins in a continuous stream back to the heart. 



Another function which is served by this muscular tissue is to 

 regulate the amount of blood which flows through the capillaries of 

 any organ in proportion to its needs. During digestion, for instance, 

 it is necessary that the digestive organs should be supplied with a 

 large quantity of blood: for this purpose the arterioles of the 

 splanchnic area are relaxed, and there is a vast amount of blood in this 

 area, and therefore a correspondingly small amount in other areas, such 

 as the skin ; this accounts for the sensation of chilliness experienced 

 after a full meal. The skin vessels form another good example ; one 

 of the most important uses of the skin is to get rid of the heat of 



