

CH.XXI.] VASO-MOTOR NERVES. 297 



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 discovered by Schiff (1855), and more accurately 

 localised by Ludwig (1871). The dilatator 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, Gaskell, Langley, and Ramon 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 in order to force the blood in a con- 

 tinuous stream through the capillaries and veins 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 correspond- 

 ingly small amount in other areas, such as the skin ; this 

 accounts for the sensation of chilliness experienced after a full 

 nidil. The skin vessels form another good example ; one of 

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

 the body in such a way that the body temperature shall remain 

 constant ; when excess of heat is produced there is also an 

 increase in the loss of heat; the skin vessels are then dilated 

 and so more blood is exposed on the surface, and thus increase 

 in the radiation of heat from the surface is brought about. On 

 the other hand, when it is necessary that the heat produced 

 should be kept in the body, the loss of heat is diminished by a 

 constriction of the skin vessels, as in cold weather. The alteration 

 of the calibre of the vessels is brought about by the action of the 

 vaso-motor nervous system on the muscular tissue of the arterioles. 



There are certain organs of the body in which the necessity for 

 alterations in their blood supply does not exist. Such organs are 

 the lungs and the brain. It is in the vessels of these organs 

 that the influence of vaso-motor nerves is at a minimum. The 

 pulmonary vessels are supplied by nerves which have been 



