190 PHYSIOLOGY FOR DENTAL STUDENTS. 



parable in action to the accelerator nerves to the heart, since stim- 

 ulation of either type of nerve tends to produce an increase in the 

 blood pressure, the one by quickening the heart rate and the 

 other by constricting the blood vessels and increasing the resist- 

 ance to the flow of blood. 



The presence of the vasoconstrictor fibers in the sympathetic 

 nerves is easily shown by the fact that stimulation of these nerves 

 to any part of the body produces a marked diminution in the 

 size of the part to which the nerves are connected. At the same 

 time there is an increase in the general blood pressure, because 

 the freedom of outflow of blood from the arterial system is some- 

 what reduced. The large nerves which supply the limbs also 

 contain vasoconstrictor nerves. These are derived from fibers 

 coming from the ganglia of the sympathetic chain in the thorax 

 and abdomen and joining with the roots of the spinal nerves in 

 order that the fibers may 'be distributed along with the cerebro- 

 spinal nerves to the part in question (see p. 277). 



After section of the spinal cord, the blood vessels of the part 

 of the body supplied with vasoconstrictor nerves below the level 

 of the section of the cord, become dilated, and may be constricted 

 again if a stimulus be applied to the lower end of the cord. The 

 effect of such a stimulus is to increase the blood pressure, since 

 the resistance offered to the flow of blood is increased. 



The organ in which the changes taking place in the blood 

 vessels under various conditions can be most easily demonstrated 

 is the kidney. It is not hard to enclose one kidney in an air- 

 tight box, and by means of rubber tubing to connect the box 

 with an instrument called a tambour, which will record on a 

 smoked drum any change in the amount of air in the box, 

 i. e., any increase in kidney volume will cause air to pass out 

 of the box, or the reverse in case the kidney volume decreases. 

 The instrument is called a plethysmograph. The instrument 

 applied to the kidney of anaesthetized animals records each heart 

 beat, or, in other words, shows a pulse. Any change in blood 

 pressure will also cause a change in kidney volume, but the 

 nature of the change will depend on the cause of the change 

 of blood pressure. (See Fig. 26). 



