68 4 NERVOUS SYSTEM. 



ments, occurring without consciousness, as the result of impressions made upon the affe- 

 rent nerves and involving the independent action of the spinal cord. 



Reflex Action of the Spinal Cord. In 1832 and 1833, Marshall Hall described 

 minutely the movements which take place in decapitated animals as a consequence of 

 stimulation of the sensory nerves, and he formularized these phenomena under the head of 

 " the reflex function of the medulla oblongata and medulla spinalis." Since this publica- 

 tion, a new interest has been attached to the writings of some of the older physiologists, 

 in which reflex action, as it is now understood, had been mentioned more or less defi- 

 nitely. In the history of important advances in physiological knowledge, it has often 

 been the case that discoveries have been foreshadowed by the earlier writers ; and bibli- 

 ographical research shows that the literature of the cord as a nerve-centre forms no 

 exception to this, which is almost the rule. Some of the allusions to the cord as a centre 

 of reflex action, made anterior to 1833, are vague and indefinite ; but, on the other hand, 

 certain excito-motor actions were very accurately described by Legallois, as early as 

 1812. Marshall Hall grouped and classified these phenomena and showed their relations 

 to the cord as an independent centre ; but he has no claim to the title of the discoverer 

 of reflex action, and his experiments themselves presented little that was really new. 



The experiments of Marshall Hall, published in 1832 and 1833, are familiar to every 

 physiologist, as supplying nearly all of the omissions of previous observers. The points 

 which he assumed to have experimentally demonstrated by his researches are the follow- 

 ing: A decapitated animal, the only part of the cerebro-spinal axis which remains being 

 the spinal cord, will make no movements, if completely protected from all external im- 

 pressions. An impression made upon the sensory nerves of a decapitated animal is 

 reflected by the cord, through the motor nerves, to the muscles, and gives rise to reflex 

 movements. If the cord be destroyed, no movements follow stimulation of the surface. 

 If the centripetal and the centrifugal nerves be divided, no reflex movements can take 

 place. Experiments upon decapitated animals accord with the results of observations 

 upon acephalous foetuses and in cases of complete paraplegia from injury to the cord. 

 All of the involuntary movements observed in the healthy body are explained by the 

 theory of reflex action. These observations of Marshall Hall were, in the main, con- 

 firmed by Muller, in the year succeeding their first publication ; and, by some writers, 

 the credit of the discovery of the mechanism of reflex action is given to both Muller and 

 Marshall Hall. 



From the point of view which the present condition of science enables us to take with 

 regard to the reflex action of the cord, we have to determine the accuracy of the obser- 

 vations of Marshall Hall, and to follow out the advances that have been made by more 

 recent observers. It is important, as the first step in our inquiry, to ascertain the exact 

 condition of decapitated animals as regards their capacity for muscular movements ; and 

 upon this point there is some x difference of opinion. Marshall Hall thought that an 

 animal (a frog, for example) after decapitation, was incapable of any voluntary move- 

 ment, or of any movement which did not have, for its exciting cause, an external 

 impression. We take the example of frogs, because these are the animals most com- 

 monly used by experimenters. 



All who have experimented upon frogs have seen them jump about vigorously after 

 decapitation ; and the question whether these be spontaneous movements, so called, or 

 an excito-motor action, is more difficult to determine than would at first sight appear. 

 It would be unphilosophical to assume that, because the animal has been decapitated, the 

 movements are due to external impressions only, if we use this as evidence against the 

 possibility of spontaneous movements under these conditions. The obvious necessity of 

 the argument is to remove all possibility of external impressions or of irritation of the 

 cord itself. Upon this point, we can only speak positively from our own experiments. 

 If a frog be decapitated, so as to leave only the spinal cord intact, if we wait for from 



