THE SPINAL CORD. 189 



cutaneous branches in front and at the sides; but most of 

 the rest are united in plexuses the first four cervical form- 

 ing the cervical plexus; the last four cervical, and greater 

 part of the first dorsal, joining to make the brachial plexus 

 for the supply of the upper limb; and the lumbar and four 

 upper sacral nerves making the lumbar and sacral plexuses, 

 principally distributed to the lower limb. 



141. The functions of the anterior and posterior roots can 

 be exhibited by vivisection only. If the spinal canal of an 

 animal be laid open, and the posterior roots of the nerves 

 going to one of its limbs be divided, while the anterior roots 

 are left uninjured, the animal will continue to move the 

 limb and walk on it as if nothing had happened ; but the 

 limb may be pinched, burned, or cut, without any sign of 

 suffering being produced. The distal end of the divided 

 posterior roots may be irritated freely without any effect of 

 any sort being apparent; but if irritation be applied to the 

 end in connection with the cord, the animal will give unmis- 

 takable signs of acute pain. If, however, instead of the 

 posterior roots, the anterior roots be divided, the limb which 

 the nerves supply will become immediately powerless; the 

 animal will be no longer able to move it, and it will hang 

 flaccid; but, though paralysed in respect of motion, it will 

 exhibit no paralysis of sensation, for the animal will show 

 as much sign of pain when that limb is pinched as if the 

 nerves had not been touched. When the ends of the divided 

 anterior roots in connection with the cord are irritated, no 

 effect is produced; but when the other ends are irritated, the 

 muscles of the limb are contracted and spasmodic move- 

 ments take place. From such experiments as these, it is 

 concluded that the anterior roots of the spinal nerves are 

 motor, and the posterior roots" sensory. 



142. Experiment, which reveals thus much with regard to 

 the spinal nerves, throws light also on the functions of the 

 spinal cord. If the cord be cut right across, the animal 

 ceases to have any feeling in the regions supplied with 

 nerves given off from the part of the cord severed from com- 

 munication with the brain, and has no longer any power to 

 move them; the cord is, therefore, the sole conductor of 

 impressions to and from the spinal nerves. But it is also a 



