786 THE SPINAL CORD. 



the conductor. Such an arc, formed by the intermediation of a single 

 nerve cell between, for instance, a sensory tissue and a motile tissue, 

 may exist in simple organisms, e.g. medusa, hydra, and perhaps in the 

 viscera of vertebrates ; but the reflex arc upon which the whole complex 

 vertebrate nervous system is founded is less simple, and involves at least 

 two nerve conductors and a linkage or synapse between them, as well as 

 the recipient and the responsive tissues from which and to which respect- 

 ively the two conductors carry. The peripheral part or path of the 

 afferent conductor is one and the same, whether the arc employed be 

 simple or complex. It forms the only channel of entrance into the 

 whole nervous system. Similarly, the peripheral efferent conductor is 

 the only channel out. Hence these two ways in and out must necessarily 

 be treated introductorily to the reactions of the central nervous system. 



It was in the seventeenth century that an idea truly allied to that which 

 " reflex action " now denotes entered physiology. Some who then wrote on 

 animal physiology express it, and among them, not earliest but most 

 notoriously, Descartes. Previous to that century, certain "sympathies" 

 between parts of the body had been noted, and had latterly been referred to 

 communion established by nerves. In Cartesius' time (1650) the argument 

 had become more precise. He pointed out that many of the nerves connected 

 with the brain elicit, without affecting consciousness, involuntary movements. 

 The involuntary closure of the eyelids in result of a threatened blow was one 

 of various instances he gave. Such a process he designated a " reflected " one. 

 Boyle brought to the question the Baconian touch-stone of experiment. He 

 found the body of a viper for three days after decapitation "to be manifestly 

 sensible of punctures, being put into a fresh and vivid motion, when it lay 

 still before, upon being pricked." There is no evidence that Boyle thought 

 such movements unaccompanied by sensation. 



As to the seat of the reflexion, Descartes said the pineal gland, Willis the 

 brain, nerve plexuses, and ganglia. Next century opinion came to be grouped 

 into two camps, one maintaining the cerebro-spinal organ (Astruc, Hales, 

 Perrault, K. Boerhaave, Haller, v. Swieten, Monro, and Whytt) to be the only 

 seat of reflex action, the other the nerve plexuses and ganglia (Vieussens, H. 

 Boerhaave, Vater, Meckel, and Gasser). Hales, rector of Teddington, contri- 

 buted what is known as the fundamental experiment. He showed that the 

 movements evoked by stimuli applied to the skin of the decapitated frog are 

 lost irrevocably by destroying the spinal cord. Whytt's observations widened 

 the notion "reflex," making it include salivation and contraction of the 

 pupil. Muscular reactions he classified as — (1) produced by direct stimuli 

 applied to muscles, (2) vital and involuntary, (3) voluntary. Each of these he 

 considered accompanied by sensation. To Hales' fundamental observation he 

 contributed a fundamental one on inhibition, noting that the reflex move- 

 ments are inelicitable for the interval immediately succeeding the decapitation. 

 Here again he invoked sensation, attributing the arrest to pain or fright. The 

 part that Prochaska took in the development of the study appears over- 

 estimated ; his experimental contributions are negligible; he held, however, 

 like Descartes, that some reflex actions do not involve sensation. 



The Bell-Magendie experiment was the next step, and its importance can 

 hardly be overstated. The localisation of the ingoing channel in one root, and 

 the outgoing in the other, was followed by the discoveries by Mayo (1823) of a 

 small circumscript segment of the brain, which of itself, and alone, sufficed 

 for the pupil reflex, and by Legallois (1826) of a small circumscript portion 

 of the bulb, injury of which at once paralyses the respiratory movements, 

 whereas other injuries do not. The "localisation" was carried further 

 by Flourens and Longet. Prior to the work of these last, Marshall 



