BEFLEX ACTION OF THE SPINAL CORD. 309 



movements may, consequently, be produced by stimulating 

 the sensory nerves in their course, or by irritating the poste- 

 rior roots of the spinal nerves. 



TTe have already stated that the cord must retain its 

 anatomical integrity, in order to receive an impression made 

 upon the centripetal nerves, and transform it, as it were, into 

 a stimulus, which is reflected back by the motor nerves and 

 produces muscular contraction. It is also evident ' that the 

 motor nerves must retain their connection with the cord, 

 and be in a condition to conduct the stimulus reflected by 

 the cord to the muscles. 



The reflex excitability of the spinal cord is increased to 

 a marked degree by separating this portion of the cerebro- 

 spinal axis from the encephalon, and the same is true for the 

 lower portion of the cord, when a section is made in the dor- 

 sal or the lumbar region. It is difficult to find an entirely 

 satisfactory explanation of this fact ; and the phenomena ob- 

 served under these conditions are, in this regard, like the 

 exaggerated sensibility of portions of the general surface 

 after section of certain columns of the cord. Setschenow 

 proposed, some years ago, the theory that the reflex excita- 

 bility of the cord under natural conditions was subject to a 

 moderating, or an inhibitory influence from the encepha- 

 lon ; and that this influence being absent in decapitated ani- 

 mals, the excitability of the cord, under these conditions, 

 seemed to be exaggerated. 1 Whether this explanation be 

 accepted or not, the fact remains, that reflex phenomena 

 are more easily excited and are more marked in animals 

 after decapitation, than in the same animals, when the con- 

 nections between the cord and brain have not been de- 

 stroyed. In addition, Yulpian has shown that the excita- 

 bility is intense in proportion as the part of the cord con- 



1 SETSCHENOW, Physiologische Studien uber die Hemmungsmechanismen fur 

 die Rfflexthatigkeit des Ruckenmarks im Gehirne des Frosches, Berlin, 1863 ; and, 

 SETSCHENOW UND PASCHTTTIN, Neue Versuche am Him und Riickenmark, Berlin, 

 1865. 



