590 NERVOUS SYSTEM. 



in short, it undergoes those changes observed in all nerves separated from their centres. 

 On the other hand, in the posterior roots, the end attached to the cord undergoes degen- 

 eration, and the peripheral end, the one to which the ganglion is attached, preserves its 

 normal histological characters. From these experiments, which have been confirmed and 

 somewhat extended by Bernard, it is concluded that the ganglia of the posterior roots 

 have an influence over the nutrition of the sensitive nerves, in the same way as the cen- 

 tres influence the nutrition of the motor nerves with which they are connected. These 

 points are interesting, as showing the existence of centres attached to the sensory system 

 of nerves, which have, as far as we know, a purely trophic influence over the nerves, 

 while the centres to which the motor nerves are attached regulate, to a certain extent, 

 the nutrition of the nerves, and also are capable of generating nerve-force. We do not 

 know that the ganglia of the roots of sensitive nerves have any function except that 

 which has just been indicated. 



Properties of the Anterior JKoots of the Spinal Nerves. The same experiments that 

 demonstrated that the posterior roots of the spinal nerves are sensitive showed that the 

 anterior roots are motor. If the two roots be exposed in an animal just killed, no con- 

 vulsive movements are produced by stimulating the posterior roots ; but, if the anterior 

 roots be irritated, movements of the most violent character occur, confined to those 

 muscles to which the filaments of the roots are distributed. There has never been any 

 doubt upon this point since the experiments of Magendie ; and it is now universally 

 admitted by physiologists, that the motor properties of the mixed nerves are derived 

 exclusively from their anterior roots of origin from the spinal cord. The question has 

 arisen, however, whether the anterior roots be not also endowed with sensibility, nota- 

 bly less in degree than the posterior roots, but still marked and invariable. The sensi- 

 bility observed in the anterior roots is abolished by section of the posterior roots ; and 

 this property, which is thought to be derived from the posterior roots, has been called 

 recurrent sensibility. 



Recurrent Sensibility. The experimental facts with regard to the recurrent sensi- 

 bility of the anterior roots of the spinal nerves are very simple. If the two roots of a 

 spinal nerve be exposed, and if the animal be allowed to recover, by a few hours' repose, 

 from the shock of the operation, irritation of the posterior root will produce pain and 

 the general movements incident to it, but no localized contractions of muscles ; and irri- 

 tation of the anterior root will produce contraction of certain muscles and a certain 

 amount of pain, always less, however, than the pain resulting from stimulation of the 

 posterior roots. If the anterior root be divided, the end attached to the cord will be 

 found completely insensible, but the peripheral end will manifest the same sensibility as 

 the undivided root ; showing that the sensory properties of the anterior roots are not 

 derived from the cord. If the posterior root be divided, the sensibility of the anterior 

 root is instantly abolished; showing that the sensibility of the anterior root is recurrent, 

 being derived from the posterior root through the periphery. "With regard to these 

 facts, which were first noted by Magendie, there can be no doubt, and we ourselves veri- 

 fied them in a series of experiments published in 1861. Experiments have simply 

 demonstrated the fact that the recurrent sensibility comes through the periphery, with- 

 out actually showing any recurrent fibres ; and division of the mixed nerve beyond the 

 point of union of the two roots deprives the anterior root of its sensibility, showing 

 that the recurrent fibres, if they exist, must turn back near the periphery. 



The question now arises with regard to the exact mechanism of recurrent sensibility. 

 The explanation offered by Magendie and Bernard is, that there are actually fibres return- 

 ing from the posterior to the anterior roots ; that these fibres are, of course, sensitive ; 

 and that irritation of the anterior roots is propagated toward the periphery and returns 

 to the centres through the posterior roots. This explanation satisfies all of the experi- 

 mental conditions, and it is farther sustained by the microscopical examinations of Schiff 

 and of Philipeaux and Vulpian. It will be remembered that the ganglia of the posterior 



