396 



PRACTICAL PHYSIOLOGY 



respectively. If the plethysmograph be connected with the supply 

 tube to a small gas jet the pulse is communicated to the flame and may 

 be photographed on a moving sensitised plate. The plate is run on a 

 traveller behind the slit in the dark room. The curves thus recorded 

 are similar to the velocity curves obtained with Chauveau's haemo- 

 dromograph, for the change of volume in the limb, if the venous 

 outflow be constant, follows the velocity of the arterial inflow. 



FIG. 256. Limb plethysmograph. 



A Method of Exposing the Spinal Roots for the purpose of Excita- 

 tion as in investigations of Vaso- Viscero- and Pilo-motor Nerves. 

 An incision three inches long is made over the spine. The muscles 

 (erector spinae) are rapidly divided on either side of the spinous 

 processes and pulled aside. The haemorrhage is then arrested by 

 pressure with wool pads or artery forceps. The transverse processes 

 and ribs are thus clearly exposed. The spinous processes and neural 

 arches are next removed with the bone forceps and the spinal cord 

 enclosed in the dura mater exposed. Pads of cotton-wool are now 

 packed into the wound and left until all haemorrhage is arrested. The 

 dura mater is then opened and the posterior roots are gently lifted up, 

 ligatured in two places, and divided between the ligature. The 

 ligatured ends are then ready for excitation. The anterior roots are 

 ligatured and divided close to the cord. After division of the posterior 

 root, the entire nerve may be excited outside the dura mater in order 

 to demonstrate the efferent effects. The presence of efferent vaso- 



