274 SPECIAL PHYSIOLOGY. 



does not originate voluntary motorial stimuli, nor effectually feel sen- 

 sorial impressions. But, in regard to motion in general, we shall find 

 that the cord is a governing centre for a particular class of involun- 

 tary movements, which have repeatedly been mentioned, viz., the reflex 

 movements ; and, in reference to sensory impressions, we shall find 

 that the cord, when in a state of integrity, can not only conduct such 

 impressions to the common sensorium, but may also transfer them 

 from nerve to nerve, and cause them to radiate from one branch of the 

 same nerve to other branches. 



The transference of sensation from one nerve to another, through 

 the cord, is illustrated by the occurrence of pain in the knee-joint, in 

 cases where the disease is actually in the hip, or by the feeling of pain 

 in the heel, when the kidney is the seat of irritation, or by the pains 

 felt in the limbs, in certain examples of disease of the brain. In these 

 cases, the sensory impressions are said to be transferred from nerve to 

 nerve, through their connecting bond of gray substance in the cord ; 

 for they have only been noticed where the nerves retain their connec- 

 tion with their proper gray centres. The radiation of sensations from 

 one branch of a nerve to another, is illustrated in the case of neuralgic 

 affections proceeding from a local injury to one branch of a nerve, say 

 of the skin of the hand, and also when a nerve entangled in a cicatrix, 

 or in the sides of a growing tumor, causes pain, not only in the part, 

 but also along the track of other branches of the same nerve ; these 

 phenomena are observed only so long as the nerves are in connection 

 with their gray centres. It is open to question, whether a very power- 

 ful stimulus to one nerve-fibre, may excite adjacent fibres, as, for ex- 

 ample, in the interlacing parts of a plexus, and so produce pain refer- 

 able to distant parts ; we have elsewhere seen, that a stimulus applied 

 to one nerve, may communicate itself to a neighboring nerve (p. 229). 

 This occurs more commonly, when the nervous centre connecting the 

 nerves, is in a particular condition of excitement. Thus, in a highly 

 exalted state of the nervous excitability, as, for example, in the con- 

 dition of inflammation, or of irritation produced by strychnine, the 

 ordinary insulation of the nerve-fibres may, as it were, be broken 

 through, and then stimuli applied to one set of fibres may excite adja- 

 cent ones directly, without the intermediation of the gray matter of 

 their common nervous centre. 



It remains to notice the reflection of impressions, brought by affer- 

 ent fibres to the cord, upon efferent fibres proceeding from the cord, 

 producing what are called reflex actions or movements, and constituting 

 what is known as the reflex excito-.motory power of the cord. The ex- 

 istence of such a controlling or regulating power in the cord, over the 

 muscles supplied by nerves issuing from it, is shown by the fact, that 

 a decapitated lizard or frog will remain standing on its feet, and will 

 manifest special movements, if the skin be irritated. On further 

 division and subdivision of the trunk and tail, each segment of the 

 lizard, containing its portion of the spinal cord, still continues to ex- 

 hibit similar movements. Again, when the spinal cord of a frog is 

 divided, all voluntary motion, as we have already seen, ceases in the 

 parts below. If the posterior columns of the severed portion of the 



