SENSIBILITY AND EXCITABILITY IN SPINAL CORD. 403 



follows ; but if the separated extremity (c) be irritated, convulsive 

 movements instantly take place. The nervous force, consequently, 

 travels in the anterior root from within outward, but cannot pass 

 from without inward. 



The same thing is true with regard to the transmission of sensa- 

 tion and motion in the spinal nerves outside the spinal canal. If 

 one of these nerves be divided in the living animal, and its attached 

 extremity irritated, pain is produced, but no convulsive motion ; if 

 the irritation be applied to its separated extremity, muscular con- 

 tractions follow, but no painful sensation. 



There are, therefore, two kinds of filaments in the spinal nerves, 

 not distinguishable by the eye, but entirely distinct in their charac- 

 ter and function, viz., the "sensitive" filaments, or those which 

 convey sensation, and the " motor" filaments, or those which excite 

 movement. These filaments are never confounded with each other 

 in their action, nor can they perform each other's functions. The 

 sensitive filaments convey the nervous force only in a centripetal, 

 the motor only in a centrifugal direction. The former preside over 

 sensation, and have nothing to do with motion ; the latter preside 

 over motion, and have nothing to do with sensation. Within the 

 spinal canal the two kinds of filaments are separated from each 

 other, constituting the anterior and posterior roots of each spinal 

 nerve; but externally they are mingled together in a common 

 trunk. While the anterior and posterior roots, therefore, are ex- 

 clusively sensitive or exclusively motor, the spinal nerves beyond 

 the junction of the roots are called mixed nerves, because they con- 

 tain at the same time motor and sensitive filaments. The mixed 

 nerves accordingly preside at the same tim over the functions of 

 movement and sensation. 



DISTINCT SEAT OF SENSIBILITY AND EXCITABILITY IN THE 

 SPINAL CORD. Various experimenters have demonstrated the fact 

 that different parts of the spinal cord, like the two roots of the 

 spinal nerves, are separately endowed with sensibility and excita- 

 bility. The anterior columns of the cord, like the anterior roots of 

 the spinal nerves, are excitable but not sensitive; the posterior 

 columns, like the posterior roots of the spinal nerves, are sensitive 

 but not excitable. Accordingly, when the spinal canal is opened 

 in the living animal, an irritation applied to the anterior columns 

 of the cord produces immediately convulsions in the limbs below ; 

 but there is no indication of pain. On the other hand, signs of 



