ADVANCED EXPERIMENTAL PHYSIOLOGY 175 



left intact. After the removal of the ilium the nerves of the sacral 

 plexus can be easily found and followed up to the spinal cord. 

 Starting from the top of the urostyle the laminae of the vertebrae are 

 carefully removed by scissors, the points of which should not be 

 plunged deeply inwards, otherwise the spinal cord will be injured. 

 After the removal of several laminae one of the large nerves of the 

 sacral plexus is followed up to its intervertebral foramen, where a 

 black swelling about the size of the head of a pin will be seen. This 

 is the posterior root-ganglion. It is freed from the foramen by 

 careful dissection, and the roots are traced therefrom to the spinal 

 cord. Fine threads are placed under the roots, which are then 

 divided in the middle of their length by clean sharp scissors. 



Stimulation of the peripheral end of the motor root will cause a 

 contraction of the muscles of the corresponding leg ; stimulation of 

 the central end with a weak induction shock will cause no movement. 

 On the other hand, stimulation of the peripheral end of the posterior 

 root produces no movement, but a similar stimulus applied to the 

 central end sets up a sensory impulse which produces reflex move- 

 ments. 



The roots of the spinal nerves are longest in the lower segments of 

 the spinal cord ; for this reason the experiment is most readily per- 

 formed in this region. During development the vertebral column 

 grows more quickly than the spinal cord, and thus the lower pos- 

 terior root ganglia in the intervertebral foramina are separated 

 from the spinal cord by a longer length of nerve-roots than in the 

 case of those supplying the upper limb. 



CHAPTER XXXVII 



INVESTIGATION OF THE MOTOR FUNCTIONS OF THE 

 ALIMENTARY CANAL BY MEANS OF THE X-RAYS 



By ARTHUR F. HURST, M.A., M.D., F.R.C.P., Physician, late 

 Demonstrator of Physiology, Guy's Hospital. 



The soft viscera are transparent and the salts of the heavy metals 

 are opaque to the X-rays. When therefore any part of the 

 alimentary canal contains food mixed with such a salt, it casts a 

 shadow on the fluorescent screen, when X-rays pass through the body. 

 Barium sulphate is the most useful for this purpose, as it is unaffected 

 by the hydrochloric acid of the gastric juice, and passes through the 

 alimentary canal without influencing its motor functions in any way, 

 and it is much cheaper than the bismuth salts which were formerly 

 used. 



A small breakfast should be taken on the morning of the examina- 

 tion in order that the stomach may be as empty as possible when the 

 opaque meal is eaten. Half a pint of bread and milk or porridge 

 mixed with three ounces of barium sulphate forms the meal. A 



