PHYSIOLOGY OF THE NERVOUS SYSTEM. 



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portnnt to ascertain what is the mechanism by 

 which these various actions take place. 



The most convenient way to discuss this 

 point will be to examine into the value of cer- 

 tain hypotheses which have been framed to 

 explain it. We shall find it necessary in this 

 discussion to keep before us two propositions 

 in favour of which sufficient evidence has al- 

 ready been adduced. These are, 1. That the 

 brain or some part of it is essential to the 

 production of mental nervous actions ; in other 

 words, that acts of volition and sensation cannot 

 take place without the brain: and, 2. That the 

 vesicular is the truly dynamic nervous matter, 

 that which is essential to and the source of the 

 developement of all nervous power. 



The first hypothesis which we shall notice is 

 one of so much ingenuity that one is tempted 

 thereby to adopt it, and would gladly do so if 

 it were found sufficient to explain the pheno- 

 mena, and if it were consistent with that sim- 

 plicity which characterises the mechanism of 

 the body. It originated with Dr. Marshall 

 Hall, and has been advocated by him with great 

 zeal and ability; it may be distinguished as 

 the hypothesis of an excito-motory system of 

 nerves, and of a true spinal cord, the centre of 

 all physical nervous actions. 



Tins hypothesis may be stated as follows.* 



The various muscles and sentient surfaces of 

 the body are connected with the brain by nerve 

 fibres which pass from the one to the other. 

 Those fibres destined for or proceeding from 

 the trunk to the brain pass along the spinal 

 cord, so that that organ is in great part no 

 more than a bundle of nerve fibres going to 

 and from the brain. These fibres are specially 

 for sensation and volition sensori-volitional. 



* I am very desirous that this hypothesis should 

 be stated correctly, as I consider that both physio- 

 logy and practical medicine are greatly indebted to 

 Dr. Marsluill Hall for the attention his labours 

 have awakened to the inherent powers of the ner- 

 vous system. Nevertheless I have shown in the 

 text that great advances in our knowledge of these 

 powers had been made by certain physiologists of 

 the last century, whose views and researches had 

 been completely or almost forgotten. 



I have collected the statement of Dr. Hall's 

 hypothesis in the text chiefly from his later wri- 

 tings. The history of Dr. Hall's labours on this 

 part of physiology, as I gather it from his writings, 

 appears to be as follows : 



In 1832 a paper was presented by him to the 

 Zoological Society, of which, so far as I can ascer- 

 tain, no other record has been kept than that which 

 is to be found in the printed summary of the Proceed- 

 ings of that Society. 1 have not had any opportunity 

 of consulting these proceedings, but find an extract 

 from them printed in Dr. Hall's work entitled Me- 

 moirs of the Nervous System, published in 1837. 

 This paper was entitled, " A brief Account of a par- 

 ticular Function of the Nervous System," and its 

 object was to point out the existence of a source of 

 muscular action distinct from all those hitherto no- 

 ticed by physiologists. The peculiarity of this mo- 

 tion is stated to consist in its being excited by irrita- 

 tion of the extreme portion of the sentient nerves, 

 whence the impression is conveyed through the 

 corresponding portion of the brain and spinal marrow 

 as a centre, to the extremities of the motor nerves. 

 Experiments upon salamanders, frogs, and turtles 

 were detailed, from which Dr. Hall drew the follow- 



But, in addition to these, there is, according 

 to Dr. Hall, another class of fibres proper to 



ing conclusions : 1. that the nerves of sensibility 

 are impressible in portions of an animal separated 

 from the rest ; in the head, in the upper part of 

 the trunk, in the lower part of the trunk ; 2. that 

 motions similar to voluntary motions follow these 

 impressions made upon the sentient nerves; and, 

 3. that the presence of the spinal marrow is essen- 

 tial as the central and cementing link between 

 the sentient and motor nerves. 



Other experiments were detailed in this paper 

 upon frogs rendered tetanic by a solution of opium; 

 thee showed that in this state the cutaneous nerves 

 became " extremely susceptive, and the motor 

 nerves extremely excitative." Decapitation of a 

 tetanized frog did not destroy the tetanic condition 

 of the trunk and extremities. " The exalted con- 

 dition of the function of the sentient and motor 

 nerves continued in each part." " All was changed 

 in removing the brain and the respective portions 

 of the spinal marrosv." 



" These experiments," Dr. Hall continued, " ap- 

 pear to me to establish a property or function of 

 the nervous system, of the sentient and motor 

 nerves, distinct from sensation and voluntary or in- 

 stinct ice motion." 



Dr. Hall's next publication appears to have 

 been a paper read before the Royal Society on 

 the 20th of June, 1833. This paper is entith-d 

 " On the reflex function of the medulla oUoni;nt i 

 and medulla spinalis." Having noticed the con- 

 clusion arrived at by Le Gallois, and confirmed by 

 the reporters of the Institute, that section of the 

 spinal marrow in the neck arrests only the respira- 

 tory movements, leaving sensation and voluntary 

 motion to remain in the whole body, he points out 

 that the causes of muscular motion may be centric 

 or eccentric in the nervous system. When the 

 cause is eccentric, that is, distant from the nervous 

 centres, Dr. Hall states that the phenomena are 

 due to a peculiar function, which he considers 

 had not previously been understood. Its charac- 

 teristic is that it is " excited in its action and reflex 

 in its course ; in every instance in which it is ex- 

 cited, an impression made upon the extremities of 

 certain nerves is conveyed to the medulla oblon- 

 gata or medulla spinalis, and is reflected along 

 other nerves to parts adjacent to, or remote from, 

 that which has received the impression." 



" It is by this reflex character," he adds, " that 

 the function, to which 1 have alluded, is to be 

 distinguished from every other." Yet, curious to 

 say, he assigns to it powers which certainly cannot 

 be excited in the reflex manner. He says, " the 

 reflex function exists as a continuous muscular 

 action, as a power presiding over organs not ac- 

 tually in a state of motion, preserving in some, as 

 the glottis, an open, in others, as the sphincters, a 

 closed form, and in the limbs a due degree of equi- 

 librium or balanced muscular action, a function 

 not, I think, recognised by physiologists." 



Dr. Hall points out the dislinctness of this func- 

 tion from sensation and voluntary motion, and re- 

 lates experiments on decapitated animals, (snakes, 

 turtles, vipers, toads, frogs, and efts,) to show 

 that the motions which occur in them are not spon- 

 taneous but only excited, and that these " excited 

 motions in decapitated animals are dependent upon 

 a principle different from sensation and volition." 

 He then shows the difference between the reflex 

 movements and those arising from instability, by 

 comparing the motion of the heart, when touched, 

 with that of the glottis of an animal when similarly 

 stimulated. Both movements take p'ace equally 

 after the removal of the brain ; but if the medulla 

 oblongata be removed, the contractions of the 

 larynx cease, while those of the heart continue. 

 " The difference consists, then, in the presence of 

 the medulla oblongata, which is essential to the 



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