566 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



die in 1822 published the results of his investigations, to be discussed imme- 

 diately, Bell came over to that author's position. The law which Magendie 

 established is often known even yet as Bell's doctrine. 



Magendie s doctrine is that the anterior roots of the spinal nerve contain 

 only efferent fibers, the posterior roots only afferent fibers. 



Originally the demonstrations by Magendie, Bell, Johannes Miiller and their 

 followers only applied to the motor nerves in the strict sense. But after other 

 efferent nerves had been discovered, proof was soon forthcoming that they also 

 make their exit by the anterior roots. Proof for the vasoconstrictors was fur- 

 nished by Pfliiger and Claude Bernard, for the vasodilators by Dastre and Morat 

 also Gaskell, and for the sweat nerves by Luchsinger. 



Several very noteworthy exceptions to this general law have now to be ad- 

 mitted. Thus the posterior roots do not contain afferent fibers exclusively, but 

 also a few efferent fibers. Strieker and his pupils find vasodilator nerves for 

 the posterior extremity of the dog in the posterior roots of the fourth and fifth 

 lumbar nerves, and for the anterior extremity in the posterior roots of the 

 brachial plexus (cf. page 235). According to Steinach the posterior roots of 

 the second to the sixth spinal nerves of the frog contain motor fibers for the 

 oesophagus, stomach, and small intestine those of the sixth and seventh, motor 

 fibers for the rectum and those of the seventh to the ninth, the same for the 

 bladder; but Dale was unable to confirm these findings. In exceptional cases 

 Horton- Smith and Dale found fibers for individual skeletal muscles of the frog 

 in the posterior roots. 



Bayliss has used the method of degeneration (cf. page 567) for tracing out 

 the vasodilators contained in the posterior roots and has found that they origi- 

 nate in the spinal ganglia. Hence they also constitute a definite exception to 

 the rule. 



Still another exception, which, however, is only apparent, has been observed. 

 Magendie himself noticed that sometimes the anterior roots were sensitive. 

 Later he found that this sensibility could only be demonstrated so long as the 

 posterior roots were intact. It is now known that this sensibility is to be ac- 

 counted for by the passing of fibers from the posterior root along the anterior 

 root to sensory endings in the membranes of the cord. For this reason the sen- 

 sibility of the anterior root is called recurrent sensibility. 



It was long supposed that the sensory fibers of a mixed nerve originate only 

 in the same pair of roots as the motor fibers. Clinical observation has shown, 

 however, that the transition is much more widespread, since afferent nerves in 

 the periphery of the body often pass from one nerve trunk to another, so that 

 within certain limits sensory transmission may take place in both directions 

 within the same nerve trunk. 



4. FUNCTIONS OF THE NERVE CELL 



From the time nerve cells were first discovered it has been assumed almost 

 universally that they constitute the seat of the central functions of the nervous 

 system. The most weighty support for this view lay in the constant difference 

 of behavior between the peripheral and central systems, the difference being 

 referred almost as a matter of course to the one element which was found 

 to be specific for the central system. In view of more recent discoveries on 

 the finer structure of the central system, it is not impossible that the extra- 

 cellular net and the connections between neurofibrils play a still more im- 



