EXCITABILITY OF THE SPINAL COED. 645 



also regarded as due to tonus. This result is due to the fact that, during the activity of the 

 intact muscles, the other ones have not sufficient power to restore the parts to their normal 

 median position. The following experiment of Auerbach and Heidenhain is against the 

 assumption of a tonic contraction : If the muscles of the leg of a decapitated frog be stretched, 

 it is found that they do not elongate after section of the sciatic nerve, or after it is paralysed by 

 touching it with ammonia or carbolic acid. 



Reflex Tonus. If, however, a decapitated frog be suspended in an abnormal position, we 

 observe, after section of the sciatic nerve, or the posterior nerve-roots on one side, that the 

 leg on that side hangs limp, while the leg of the sound side is slightly retracted. The sensory 

 nerves of the latter are slightly and continually stimulated by the weight of the limb, so that 

 a slight reflex retraction of the leg takes place, which disappears as soon as the sensory nerves 

 of the leg are divided. If we choose to call this slight retraction tonus, then it is a reflex tonus 

 (Brondgeest). (See the experiments of Harless, G. Ludwig, and Cyon 355. ) 



363. EXCITABILITY OF THE SPINAL CORD. Even at the present 

 time observers are by no means agreed whether the spinal cord, like peripheral 

 nerves, is excitable, or whether it is distinguished by the remarkable peculiarity that 

 most of its conducting paths and ganglia do not react to direct electrical and 

 mechanical stimuli. 



It is contended by some observers that if stimuli be cautiously applied either to white or 

 grey matter, there is neither movement nor sensation ( Van Deen (1841), Brown- Seqicard). Care 

 must be taken not to stimulate the roots of the spinal nerves, as these respond at once to 

 stimuli, and thus may give rise to movements or sensations. As the spinal cord conducts to 

 the brain impulses communicated to it from the stimulated posterior roots, but does not itself 

 respond to stimuli which produce sensations, Schiff has applied to it the term " aesthesodic. " 

 Further, as the cord can conduct both voluntary and reflex motor impulses, without, however, 

 itself being affected by motor impulses applied to it directly, he calls it " kinesodic." 



SchifTs views are as follows : 



1. In the posterior columns the sensory root-fibres of the posterior root 

 which traverse these columns give rise to painful impressions, but the proper paths 

 of the posterior columns themselves do not do so. The proof that stimulation of 

 the posterior column produces sensory impressions, he finds in the fact that dilata- 

 tion of the pupil occurred with every stimulation (292). Eemoval of the posterior 

 column produces anaesthesia (loss of tactile sensation). Algesia [or the sensation of 

 pain] remains intact, although at first there may even be hyperalgesia. 



2. The anterior columns are non-excitable, both for striped and non-striped 

 muscle, as long as the stimuli are applied only to the proper paths of this column. 

 But movements may follow, either when the anterior nerve-roots are stimulated, or 

 when, by the escape of the current, the posterior columns are affected, whereby 

 reflex movements are produced. 



According to Schiff, therefore, all the phenomena of irritation, which occur when an uninjured 

 cord is stimulated (spasms, contracture), are caused either by simultaneous stimulation of the 

 anterior roots, or are reflexes from the posterior columns alone, or simultaneously from the 

 posterior columns and the posterior roots. Diseases affecting only the anterior and lateral 

 columns alone never produce symptoms of irritation, but always of paralysis. In complete 

 anaesthesia and apnoea, every form of stimulus is quite inactive. According to SchifTs view, all 

 centres, both spinal and cerebral, are inexcitable by artificial means. 



Direct Excitability. Many observers, however, oppose these views, and contend that the 

 spinal cord is excitable to direct stimulation. Fick observed movements to take place when 

 he stimulated the white columns of the cord of a frog, isolated for a long distance so as to avoid 

 the escape of the stimulating currents. Sirotinin, also, who stimulated the transverse section 

 of the frog's cord from point to point, obtained contraction of the muscles both by mechanical 

 and electrical stimuli. Biedermann comes to the following conclusions : The transverse 

 section of a motor nerve is most excitable. Weak stimuli (descending opening shocks) excite 

 the cut surface of the transversely divided spinal cord, but do not act when applied further 

 down. Luchsinger asserts that, after dipping the anterior part of a beheaded snake into warm 

 water, the reflex movements of the upper part of the cord are abolished, while the direct excita- 

 bility remains. 



3. Excitability of the Vaso-motors. The vaso-constrictor nerves, which pro- 

 ceed from the vaso-motor centre and run downwards in the lateral columns of the 

 cord, are excitable by all stimuli along their whole course ; direct stimulation of 



