300 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



about ten times in a minute ; the existence of valves prevents regur- 

 gitation, so the entire effect of the contractions is auxiliary to the 

 onward current of blood. Analogous phenomena are occasionally 

 found in other animals. 



A venous pulse is observed under the conditions previously 

 described (p. 297), when the arterioles are dilated so that the arterial 

 pulse passes through the capillaries to the veins. 



A venous pulse is also seen in the superior and inferior vena 

 cava near to their entrance into the heart ; this corresponds to varia- 

 tions of the pressure in the right auricle. When the ventricle is con- 

 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-Motor 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 ; (li) those the stimulation of 

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

 nerves. 



The muscular structure of arteries was first described by Henle 

 in 1841 ; but it was not until twelve years later that the nerves 

 supplying this muscular tissue were discovered. The names of 

 Claude Bernard, Brown-Sequard, and Schiff are specially connected 

 with this discovery. 



These nerves 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 



