FUNCTIONS OF THE SPINAL CORD 863 



segments of the cord with the attached posterior roots and the grey 

 matter was excised. Long strands of the white matter of the anterior 

 (ventral) portion of the cord were isolated, and laid on electrodes, and 

 contractions of muscles were seen to follow stimulation, even when the 

 anterior roots nearest the stimulating electrodes had been cut, and every 

 precaution taken to avoid escape of current on to the distant anterior 

 roots of the nerves supplying the muscles. Indeed, apart from direct 

 experimental evidence, the fact that the white fibres of the brain are 

 universally admitted to be excitable by artificial means would be of 

 itself almost sufficient to decide the question, for we know of no essential 

 difference between the cerebral and the spinal fibres. But the con- 

 ditions must rarely occur under which direct stimulation of white fibres 

 in their course is possible in the intact body; and the only impulses with 

 which we need concern ourselves here are those that reach the con- 

 ducting paths from grey matter in the cord itself or in the brain, or from 

 the peripheral organs. 



What sort of impulses do the various tracts of the spinal cord 

 conduct ? For the dorsal or posterior roots this question was first 

 fully answered by Magendie; for the ventral or anterior roots, 

 although with a certain degree of ambiguity, by Sir Charles Bell. 

 Bell observed that when, in an animal just killed, he mechanically 

 stimulated the anterior roots, muscular contractions were obtained 

 at each touch of the forceps. He concluded that the anterior roots 

 are motor and sensory, while the posterior roots are ' vegetative ' 

 i.e., connected with the functions of the viscera, the so-called 

 ' vegetative ' organs. But although he is often credited with the 

 discovery of the functions of the posterior roots as well, he was not 

 the first to make the decisive experiment necessary to show that they 

 are the conductors of sensory impulses. It was after Magendie's 

 discovery that only a portion of the nerves are sensitive, and that 

 there are nerves ' which are like tendons, aponeuroses, or cartilages 

 in insensibility,' that Bell formulated the law that the anterior roots 

 are purely motor, the posterior purely sensory. This law, often 

 termed Bell's Law, is more correctly denominated the Magendie- 

 Bell Law. 



When the posterior roots are divided, loss of sensation occurs in 

 the region to which they are distributed. If only one root is cut, 

 the loss of sensation is never complete in any part of the skin; and 

 Sherrington has found that the cutaneous areas of distribution of 

 consecutive nerve-roots are not perfectly independent, but to some 

 extent overlap. Stimulation of the peripheral end of the divided 

 posterior root has no effect. Stimulation of the central end gives 

 rise, if the animal be conscious, to evidences of pain, and other signs 

 of the passage of afferent impulses e.g., a rise in blood-pressure. 

 The latter may also be observed when the animal is anaesthetized. 



Referred Pain. The posterior roots contain sensory fibres not only 

 for the skin, but also for the deeper structures and the viscera. The 

 afferent fibres reach the viscera by the sympathetic, the vagus, and the 

 pelvic nerves or nervi erigentes. Clinical observations have thrown 



