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The Bell-Magendie Law refers to the discovery, in the early 1800s, that sensory nerves enter the spinal cord by way of the dorsal roots of the spinal nerves, and motor nerves exit the spinal cord by way of the ventral roots. Recognition for making the discovery was attributed jointly to Charles Bell and Fran├žois Magendie. Prior to this observation, it was held that nerves were tubular conduits that served both sense and motor functions. The discovery of functionally distinct sensory and motor nerves revealed, for the first time, clear evidence of the basic structure of the nervous system. Articulation of the physiology of the spinal reflex arc and the architecture of the nervous system in terms of the specific function of sensory and motor nerves developed directly and swiftly from this first fact of neural localization.

Charles Bell (1774-1842) was an accomplished Scottish anatomist and surgeon. In 1811, he wrote a pamphlet titled Idea of a New Anatomy of the Brain: Submitted for the Observation of His Friends. The pamphlet was privately printed and distributed to 100 friends and colleagues. In this letter, he outlined a rationale for the study of brain function, and he speculated about the location of higher mental functions in the brain. He considered that the functions of specific nerves were determined from their origin in different parts of the brain. During this discourse, he noted that spinal roots emerging from the vertebra fused together to form larger nerves. His opinion that these spinal nerve roots were functionally distinct was put to the test in a simple experiment. When he severed the posterior (dorsal) root, the muscles of the back did not convulse, but he observed a convulsion of the muscle when he touched the anterior (ventral) root.

In 1822 Fran├žois Magendie (1783-1855), a French physician and physiologist, published his findings from experiments in which he cut unilaterally some of the posterior spinal roots, anterior spinal roots, or both posterior and anterior roots. Magendie had devised a clever procedure that enabled him to cut anterior roots without damaging the posterior roots. He noted that sensation (pain) was not elicited when the severed posterior root was touched, whereas the limb moved spontaneously when the anterior root was intact. Severing the anterior roots, however, caused the limb to go flaccid, whereas sensibility remained when the posterior root was intact. Magendie concluded that the anterior and posterior roots of the nerves emanating from the spinal cord have different functions, with the posterior root pertaining to sensibility, whereas the anterior root was linked to movement.

Following the publication of Magendie's article in 1822, a challenge to the priority of the discovery was issued by Charles Bell, and subsequently by his brothers-in-law John Shaw and .Alexander Shaw in various texts and journals. Bell's unpublished 1811 pamphlet was cited as the basis for his claim to be the first to establish that sensory and motor nerves were distinct entities. This campaign to assign priority for the discovery to Bell was quite successful. Bell was lauded for the discovery by many eminent physiologists and scholars throughout the nineteenth century, such as Sher-rington, who made seminal contributions to the physiology of spinal reflex arcs; Neuberger, a respected medical historian; and even by some of Magendie's contemporary French physicians, such as Flourens. Scholars who have more recently examined documents relevant to the discovery, however, dispute Bell's claim for priority.

An analysis of the controversy was thoroughly documented by Cranefield (1974), in a text that includes a facsimile of Bell's annotated letter to his friends, as well as facsimiles of all of the material by Bell, John Shaw, and Ma-gendie on which the claim for priority can be based. Clearly, there is no challenge to Magendie's experiment that is precise, elegant, and unambiguous in demonstrating, and correctly interpreting, the sensory function of the posterior spinal root and the motor function of the anterior spinal root. Several issues were raised that cast aspersions on Bell's claim. That Bell's pamphlet was privately printed and circulated, rather than published in a scientific journal that was open to public scrutiny, certainly detracts from the authority of discovery. Second, during the period from 1816 to 1823, Bell and John Shaw published numerous articles on the anatomy of the brain and nerves, but in none of these was there a specific statement about the functions of the spinal nerve roots. This indifference is in marked contrast to the importance of the discovery claimed by Bell after Magendie's publication in 1822. Finally, following the procedure described in Idea of a New Anatomy of the Brain (Bell, 1811/1974), there was no basis for suggesting the sensory function for the anterior spinal roots.

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