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PHYSIOLOGY OF THE NERVOUS SYSTEM. 



the motor nerve to the muscles of the features, 

 and to the orbicular muscles of the eyelids. 

 Clinical research, indeed, taken in conjunction 

 with anatomy, forms the basis of our present 

 accurate knowledge of the office of this nerve. 

 In like manner we learn that loss of sensibility 

 of the face is dependent on disease affecting the 

 fifth nerve, and from the parts of the face 

 which are affected by anaesthesia we can tell 

 what portions of that great nerve are diseased. 

 Here again anatomy and clinical medicine have 

 mainly contributed to the advance of our know- 

 ledge. The partial palsies which affect the 

 muscles of the eye ball likewise give very dis- 

 tinct interpretation to the functions of these 

 nerves, such as the third and sixth, the action 

 of whose muscles is well understood. Many 

 other instances might be quoted which clearly 

 show that, while clinical medicine and anatomy 

 are of infinite service in building up and con- 

 firming our knowledge of the function of nerves, 

 this knowledge, in its turn, does great service 

 in increasing the facility with which we can 

 distinguish disease. 



Of the functions of the roots of spinal nerves. 

 The greatest part of the body is supplied 

 with nerves which are implanted in the spinal 

 cord, or which, in anatomical language, have 

 their origin in that nervous centre. As these 

 nerves present very definite and constant cha- 

 racters as regards the manner in which they are 

 connected with the centre, characters which are 

 not limited to the human subject, but which be- 

 long to all classes of vertebrate animals, it was 

 a point of primary importance to discover the 

 object of an arrangement so peculiar as regards 

 its anatomical characters, and so universal. To 

 our countryman, Sir C. Bell, belongs the great 

 merit of having seen the importance of deter- 

 mining this point as a preliminary step in the 

 investigations into the nervous system ; and to 

 him must be awarded the credit of having 

 achieved the discovery of the difference in the 

 endowment of the anterior and of the posterior 

 roots of these nerves. He experimented on 

 young rabbits, by removing the posterior wall 

 of the spinal column. "On laying bare the 

 roots of the spinal nerves," says Sir C. Bell, "I 

 found that I could cut across the posterior 

 fasciculus of nerves, which took its origin from 

 the posterior portion of the spinal marrow, 

 without convulsing the muscles of the back ; 

 but that, on touching the anterior fasciculus 

 with the point of the knife, the muscles of the 

 back were immediately convulsed." * 



Numerous experimenters, subsequent to Bell, 

 obtained precisely similar results. Muller, 



* Sir C. Bell's first essay on this subject was 

 printed in 1811. In 1822 Majendie published his 

 first essay in the Journal de Physiologie Exp. t. iii ; 

 in 1831 Miiller's experiments were published in the 

 Annales des Sciences Nat. and in Froriep's Notizen. 

 Mr. Alexander Shaw has published a temperate 

 and judicious vindication of Sir C. Bell's claims in 

 a volume entitled, " Narrative of the Discoveries 

 of Sir C. Bell in the Nervous System." Lond.1839. 



Valentin is so satisfied of Sir C. Bell's claim to 

 the discovery of the distinct endowments of the 

 roots of the spinal nerves, that he designates the 

 law thereby determined by a title not very eupho- 

 nous to English ears, Lex BeMiana. 



however, obtained the most decisive evidence of 

 the proper functions of the roots of the nerves, 

 by experimenting on frogs instead of on mam- 

 malia; in the former the spinal canal is of 

 great width, especially at its lower part, and 

 the roots of the nerves can be exposed with 

 great facility, whilst in the latter the operation 

 is tedious, painful, and bloody, the spinal 

 canal narrow, and the roots of the nerves small 

 and difficult to get at. Moreover, the excita- 

 bility of the nerves lasts very much longer in 

 frogs than in mammalia, and on this account 

 the former animals are well adapted for dis- 

 playing the effects of section of the roots and 

 the influence of mechanical and other stimuli 

 upon them. 



I n these experiments, (which I have frequently 

 repeated with similar results,) irritation, mecha- 

 nical orgalvanic, of the anterior root of the spinal 

 nerve always provokes muscular contraction. 

 No such effect follows irritation of the posterior 

 root. Section of the anterior root causes para- 

 lysis of motion ; section of the posterior root, 

 paralysis of sensation. This latter effect is 

 shown by the entire insensibility to pain evinced 

 on pinching a toe, whilst in the limb of which 

 the posterior roots of the nerves remained en- 

 tire such irritation is evidently felt acutely. If 

 the anterior roots of the nerves which are dis- 

 tributed to the lower extremities be cut on one 

 side, and the posterior roots on the oilier, 

 voluntary power without sensation will remain 

 in the latter, and sensation without voluntary 

 power in the former. 



Valentin, Seubert, Panizza, and Longet have 

 performed similar experiments on mammiferous 

 animals with precisely similar effects. 



I have never seen motion produced by irri- 

 tation of one of the posterior roots of the spinal 

 nerves still in connexion with the cord, except- 

 ing when the galvanic stimulus has been ap- 

 plied, and too strong a current has been em- 

 ployed. Valentin states that he has observed 

 motions so produced in rabbits, but not in frogs 

 and tortoises. Dr. Hall has seen them in the 

 turtle and skate. Van Deen speaks of them 

 as constantly occurring. But Muller denies the 

 power of the posterior roots to excite motion, 

 except by " traction on the cord itself." No 

 such effect ever follows any kind of stimulation 

 of the posterior root when it has been separated 

 from the cord. 



The conclusion which inevitably follows from 

 these experiments is that the anterior root of 

 each spinal nerve is motor, and the posterior 

 sensitive. 



Comparative anatomy confirms this conclu- 

 sion, by showing that a similar arrangement of 

 the roots of spinal nerves prevails among all 

 classes of vertebrate animals, and that if in any 

 particular class either the motor or sensitive 

 power predominate, there is in correspondence 

 with it a marked developement of the anterior 

 or posterior roots, The frequent occurrence, 

 likewise, of paralysis of sensation and motion, 

 as a consequence of disease within the spinal 

 canal, also tends to the same inference. 



Kronenberg finds a small nerve of commu- 

 nication between the posterior and the anterior 

 root, which is looked upon by some as being 



