THE SPINAL CORD. 401 



limited by the median line, both in front and rear. In such cases the 

 lesion is on the opposite side, above the decussation of the anterior 

 pyramids ; namely, in the upper part of the medulla oblongata, the 

 crura cerebri, the cerebral ganglia, or the hemispheres. It is most 

 frequently seated in the cerebral ganglia or the hemispheres. 



In hemiplegia from this cause, the loss of sensibility and the loss 

 of motion occupy the same half of the body, though they are not always 

 equally well marked. When the lesion, on the other hand, is in one 

 lateral half of the spinal cord, there is loss of motion on the corre- 

 sponding side of the body, and loss of sensibility on the opposite side. 

 A number of such cases have been collected by Brown-Sequard, in 

 which the situation of the injury was ascertained by post-mortem 

 examination. 



Furthermore, a distinction is made between affections involving loss 

 of motion and those accompanied by loss of sensation. The term 

 paralysis indicates more especially an impairment or abolition of the 

 power of voluntary movement ; while diminution or loss of sensibility 

 is called anaesthesia. Either of these affections may be complete or par- 

 tial ; confined to particular regions, or extending over a considerable 

 part of the body. They may be present together, as in paraplegia ; or 

 either may exist independently, as local paralysis or local anesthesia. 

 A loss of sensibility occupying one lateral half of the body is known as 

 hemiansesthesia ; and as shown above, it may be associated with hemi- 

 plegia in the same region, or the two may coexist on opposite sides. 



The Spinal Cord as a Nervous Centre. 



So far as the spinal cord is concerned in sensation and voluntary 

 motion, it acts as a medium of communication between the brain and 

 the external parts. Its complete division at any point destroys this 

 communication ; so that the commands of the will are no longer trans- 

 mitted to the muscles, and impressions made upon the integument pro- 

 duce no conscious sensation. But after such an operation motion is not 

 altogether abolished in the limbs ; and sensitive impressions, though 

 no longer perceived by the individual, are still capable of exciting 

 muscular reaction. These phenomena, which take place without the 

 intervention of the brain, result from the action of the cord as a ner- 

 vous centre, and are due to the independent properties of its gray 

 substance. 



Reflex Action of the Spinal Cord. If a decapitated frog be allowed 

 to remain at rest for a few moments, until the effects of nervous shock 

 have passed off, movement can be excited in the limbs by applications 

 made to the integument. If the skin of one of the feet be pinched with 

 .forceps, or immersed in a weak acidulated solution, the leg is immedi- 

 ately drawn up toward the body, as if to escape the source of irrita- 

 tion. If the. stimulus be of slight intensity, the corresponding leg only 

 will move ; but if it be more severe, motion will often be produced in 



the opposite limb, or even in all the limbs at once. These phenomena 



2A 



