138 PHYSIOLOGY. 



7. A few years ago, Sir Charles Bell discovered, that by 

 opening the spinal canal in a living animal, and dividing 

 the posterior roots of the nerves, the parts to which they are 

 distributed are deprived of feeling. The limb may be prick- 

 ed or lacerated in any way, without the animal manifesting 

 the least feeling, or indication of suffering, while at the same 

 time the power of motion remains. He also found that when 

 the posterior roots were divided, sensation was destroyed, 

 but motion remained. One of Majendie's experiments, is 

 interesting. Availing himself of the fact, that the introduc- 

 tion of nux vomica into the system, produces violent spasms, 

 tremors, and rigidity of the muscles : he administered it to an 

 animal, after having severed the anterior roots of the spinal 

 nerves. The consequence was, that while all the muscles, 

 whose nerves remained entire, were thrown into a state of 

 violent spasm ; those supplied with the nerves, whose roots 

 had been divided, remained unaffected. 



8. The spinal cord then serves to connect the brain, which 

 is the common centre of sensation and voluntary motion, 

 with all the sensible parts of the body, and with the muscles, 

 the instruments of motion.) We may view it in this sense, 

 as forming a channel of communication for a bundle of 

 nerves running from the brain to every part of the system 

 to carry messages from the will, and to transmit intelligence 

 back again, as if all the roads and high-ways in the country 

 should terminate in one grand post-road, connecting them 

 with the seat of government. 



9. To illustrate, suppose I seize a hot iron in my hand, 

 not knowing it to be heated ; the painful impression is im- 

 mediately transmitted along the sensiferous nerves to the 

 spinal cord, and through that to the brain. The brain takes 

 cognizance of it, and a mental determination is immediately 

 formed to drop the iron. The will sends back its command 

 through the anterior column of the spinal cord to the motific 

 nerves, distributed to the muscles, which go to the hand. 

 The muscles instantly obey ; the iron drops, and the whole 



