30 



IRRITABILITY. 



spinal marrow destroyed, the stomach was 

 violently crushed with a hammer. The heart, 

 which previously beat vigorously sixty times in 

 a minute, stopped suddenly and remained 

 motionless for many seconds. It then con- 

 tracted ; after a long interval it contracted 

 again, and slowly and gradually recovered an 

 action of considerable frequency and vigour."* 



Dr. W. C. Henry has added an argument in 

 favour of the theory of neuromyic action of ano- 

 ther kind. It is known that certain narcotics, 

 applied to nerves, destroy the vis nervosa of 

 that part. Dr. Henry found that " a solution 

 of opium injected into the cavities of the heart, 

 or introduced into the intestine, immediately 

 arrested the actions of these organs. "f It 

 seems difficult to imagine that this effect of 

 the narcotic was not produced through the 

 medium of the nervous fibriUae, the muscular 

 being defended by the internal lining of these 

 organs respectively, in the latter organ, a mucous 

 membrane. 



After much consideration given to this 

 subject, we should be disposed to conclude 

 that in the phenomena of muscular action, the 

 stimulus acts upon the nervous fibre, and that 

 the contraction is an effect and the property of 

 the muscular fibre. 



If this view be correct, we are necessarily 

 led to consider the vis insita, or muscular 

 power, in connection with the via nervosa. 

 This latter power is peculiar to certain parts of 

 the nervous system. It is not possessed by the 

 cerebrum or cerebellum, or by the ganglia; but 

 it exists in the tubercula quadrigemina, the me- 

 dulla oblongata, the medulla spinalis, and the 

 muscular nerves. The heart itself has recently 

 been observed by Burdach to contract on sti- 

 mulating the cardiac nerves by galvanism. 



We owe the discovery of the distinct limita- 

 tion of the vis nervosa, or, as he terms it, the 

 " excitabilite," to M. Flourens.J 



The following were the supposed laws 

 of action of the vis nervosa by Mailer, Bichat, 

 and Professor Muller, before I began my own 

 researches on this subject : 



Haller observes, " Irritato nervo, convulsio 

 in musculo oritur, qui ab eo nervo ramoshabet." 

 " Irritato nervo, multis musculis communi, 

 totive artui, omnes ii musculi convelluntur, qui 

 ab eo nervo nervos habent, sub sede irritationis 

 ortos. Denique medulla spinali irritata, omnes 

 artus convelluntur, qui infra earn sedem nervos 

 accipiunt ; neque contra artus, qui supra 

 sedem irritationis ponuntur." He concludes, 

 " conditio ilia in nervo, quse motum in muscu- 

 lis ciet, desuper advenit, sive a cerebro et me- 

 dulla spinali, deorsum, versus extremes nervo- 

 rum fines propagatur." And " ut adpareat 

 causam motns a trunco nervi in ramos, non a 

 ramis in truncum venire."^ 



Bichat observes, " I'inrluence nerveuse ne 

 se propage que de la partie superieure a 1'in- 

 ferieure, et jamais en sens inverse. Coupez un 



* Op. cit. p. 160. 



t Abstracts of papers read before tlie Royal 

 Society, vol. iii. p. 65. 

 J Op. cit. p. 16, &c. 

 Elementa Physiologies, Lausannae, t. iv. p. 335. 



nerf en deux, sa partie inferieure irrite'e fera 

 contracter les muscles subjacens ; on a beau 

 exciter 1'autre, elle ne determine aucune con- 

 traction dans les muscles superieurs ; de merne 

 la moe'lle, divisee transversalement et agacee 

 en haut et en bas, ne produit un effet sensible 

 que dans le second sens. Jamais 1'influence 

 nerveuse ne remonte pour le mouvement, 

 comme elle le fait pour le sentiment." * 



Lastly, Professor Muller observes, " the 

 motor power acts only in the direction of the 

 primitive nervous fibres going to muscles, or in 

 the direction of the branches of the nerves ; and 

 never backwards ;" and " all nervous fibres 

 act in an isolated manner jfroz the trunk of a 

 nerve to its ultimate branches "-\ 



It is a singular circumstance, that an esta- 

 blished fact in experimental research, an esta- 

 lished principle of muscular action in the 

 animal economy, should be without application 

 to physiology. Yet such has been the case. 

 For what is the application of the vis nervosa 

 to the explanation of the functions of the animal 

 ceconomy ? 



Before any such application could be made, 

 it was necessary that other modes of action of 

 this power should be ascertained. I have, by a 

 series of experiments, determined new laws of 

 action of the vis nervosa, and have thus been 

 enabled to make an extensive application of 

 the principle to the functions of life. 



The head of a river tortoise being separated 

 between the third and fourth vertebrae : 



1. The dorsal portion of the spinal marrow 

 was laid bare to the extent of one inch below 

 the origin of the brachial nerves ; the spinal 

 marrow was then excited by means of the 

 probe and by galvanism ; both anterior and 

 posterior extremities, with the tail, were moved. 



2. A lateral intercostal nerve was then laid 

 bare, and stimulated in the same manner ; the 

 same effects were produced as in the former 

 experiment. 



These experiments have been repeated many 

 times, and I performed them in the presence 

 of M. Serres and other gentlemen at Paris, in 

 the month of August, 1839. They establish 

 the following new laws of action of the vis 

 nervosa : 



1. That it does act in the direction from 

 branch to trunk ; 



2. That it is in a retrograde direction in the 

 spinal marrow. 



The application of these new laws to phy- 

 siology the first application of the vis nervosa 

 to physiology is very extensive, co-extensive 

 indeed with all the acts of ingestion and 

 egestion in the animal ceconomy. But it does 

 not belong to our present article to treat of this 

 important and extensive subject. We now 

 return to that of irritability in general. 



The degree of irritability is not the same in 

 every organ of the body. Haller and Nysten 

 have investigated this subject, and the follow- 

 ing are their statements respectively : 



Haller observes, " Tenacissima virium insi- 



* Anatomic Generale, 2de partie, t. iii. p. 277- 

 278. ed. 1801. 



t Handbuch der Physiologic, i. 656. 



