ELEMENTARY EXPERIMENTAL PHYSIOLOGY 221 



cord to the periphery, the latter are afferent, carrying impulses from 

 the periphery to the spinal cord. This law can be proved by experi- 

 ments upon a brainless frog, but careful dissection and manipulation 

 are necessary. 



The following are the several stages in the experiment. A small 

 pair of electrodes is made by passing the bared ends of two pieces of 

 fine insulated wire through a piece of cork, and the induction-coil is 

 arranged for single shocks. The cerebrum of a large frog is destroyed 

 by compression with a pair of Spencer Wells forceps, and then the frog 

 is placed belly-downwards upon a cork board, and is confined to this 

 position by a piece of wet flannel fastened down tightly by pins. A 

 slit is made through the flannel in the line of the vertebral column, and 

 the skin is reflected as far as the end of the urostyle. The ilium is 

 carefully removed on one side, care being taken to avoid cutting any 

 large blood-vessels, for loss of blood would lower the excitability of the 

 spinal cord and obscure the dissection. For a similar reason the 

 medulla oblongata, which contains the vaso-motor centre, was 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 con- 

 traction 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 movements. 



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 posterior 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. 



