PHYSIOLOGY OF THE NERVOUS SYSTEM. 



720T 



which each part of this great system takes in 

 the production of nervous phenomena. This 

 inquiry naturally divides itself into twobranches, 

 namely, first, the functions of nerves; secondly, 

 those of nervous centres. 



Of the functions of nerves. Nerves are in- 

 ternuncial ; they possess in themselves (sepa- 

 rate from the nervous centre) only a very limited 

 power of developing the nervous force, and that 

 only in response to a physical stimulus, for 

 connection with a centre is necessary for the 

 exercise of a mental stimulus. 



In inquiring into the function of any parti- 

 cular nerve, the problem is to determine whether 

 itpropagates the nervous force centrad or periphe- 

 rud, and whether it be connected with the centre 

 of sensation or with the centre of volition; whe- 

 ther, in short, it be sensitive or motor. It must 

 be always borne in mind that most nerves con- 

 tain nerve-fibres of different endowments, and 

 that the office of any given nerve will be deter- 

 mined by the endowment of the greatest por- 

 tion of its fibres. When we say, therefore, that 

 a nerve is motor or sensitive, it is not to be un- 

 derstood that all its fibres are exclusively of that 

 function, and that it contains no others of a dif- 

 ferent endowment. 



In enquiring into the function of a nerve, 

 the first point to determine is its anatomy, where- 

 by we learn whether it be distributed to mus- 

 cular parts or to sentient surfaces ; and then to 

 ascertain whether its distribution in man corre- 

 sponds with that in the inferior animals. Ana- 

 tomy, human and comparative, affords by far 

 the most certain grounds to enable us to decide 

 upon the endowment of a nerve : if the nerve 

 be distributed to muscular parts, it is evident 

 that it cannot be a merely sentient nerve, 

 although it may contain some sentient fibres. 



Experiment upon animals recently dead also 

 affords considerable aid in reference to questions 

 of this kind. Mechanical or chemical or gal van ic 

 irritation of a nerve will cause muscular contrac- 

 tions if it be a motor nerve, and will produce 

 no perceptible change in either nerve or muscles 

 if it be not muscular. Under certain circum- 

 stances, however, simple irritation of a nerve, 

 while it evinces no change in the nerve itself or 

 in the parts with which it is connected, will affect 

 the portion of the nervous centre in which it is 

 implanted, and will through that excite certain 

 motor nerves to stimulate their muscles. To 

 affect motor nerves through sensitive ones, it is 

 generally necessary to stimulate their peripheral 

 fibres, the entire trunk remaining uninterrupted 

 in its course; and it would appear as if a cer- 

 tain peripheral organisation, as for instance a 

 developement of papillae on the tegumentary 

 surface, were necessary for this purpose. Very 

 rarely irritation of the trunk of a sentient nerve 

 produces this effect; the least equivocal in- 

 stance indeed in which, so far as I know, 

 muscular action can be produced in this way, 

 i. e , by irritation of the central segment of the 

 trunk of a nerve, is in the case of the glosso- 

 pharyngeal nerve. Dr. John lleid has suc- 

 ceeded, after section of this nerve, in producing 

 contraction of the pharyngeal muscles by stimu- 

 lating its central segment. 



MM. Longet and Matteucci affirm that a 



motor nerve may be distinguished from a com- 

 pound one by the different effect produced on 

 each by opening or closing a galvanic current, 

 according to the direction in which it passes in 

 the nerve. We have referred above to the 

 results of experiments on compound nerves, the 

 sciatic for instance, by means of the electric 

 current. Compound nerves, as has been shown 

 by these means, may at first be affected equally 

 on opening as on closing the circuit, whether 

 the current be direct or inverse ; but after a 

 time they are excitable, as shown by the con- 

 traction of the muscles below the point stimu- 

 lated, only on closing the, direct current or 

 opening the inverse. With a purely motor 

 nerve, however, such as the anterior root of a 

 spinal nerve, a diffetent result is obtained after 

 the first period has passed; inasmuch as the 

 contractions of the muscles can only be excited 

 on opening the direct current or closing the in- 

 verse* 



Experiment upon living animals likewise 

 affords us some assistance in determining the 

 functions of nerves. This mode of inquiry, how- 

 ever, mustbe used with great circumspection, and 

 great caution must be observed in the inter- 

 pretation of the results which it elicits. Section 

 of a nerve paralyses its function, and occasions 

 loss of motor or of sensitive power, according 

 to the nature of the parts to which the nerve is 

 distributed. Experiment of this kind, however, 

 frequently leads to very unsatisfactory results, 

 because it is often a matter of extreme difficulty 

 to reach the nerve in question; the operation 

 for that purpose may involve other parts and 

 nerves as well, and sometimes it may be impos- 

 sible to divide one nerve without injuring ano- 

 ther immediately adjacent to it. Moreover, the 

 shock of a severe operation frequently produces 

 so much disturbance in the entire system of the 

 animal as to render it extremely difficult to 

 form any accurate opinion as to the effects of 

 the section of the nerve under examination. 



Clinical medicine gives very important aid 

 to physiological enquiries of this nature. 

 Disease or injury of certain nerves impairs or 

 destroys or modifies certain functions. The 

 various forms of partial paralysis, especially 

 those affecting the face, may be referred to in 

 illustration of this assertion. Thus a very dis- 

 tinct series of signs accompany disease of the 

 facial nerve or the portio dura of the seventh 

 pair; and these signs mark it very distinctly as 



* Matteucci et Longet, sur la relation qui existe 

 entre le sens du courant electrique, et les contrac- 

 tion musculaires dues a ce courant. Paris, 1844. 



It is an extraordinary circumstance that the 

 excitability of motor nerve-fibres should be mo- 

 dified by their simple juxtaposition with sensitive 

 fibres. 



I learn from a recent communication from Prof. 

 Matteucci, (May, 1847,) that he finds that etheriza- 

 tion in dogs modifies the excitability of the nerves, 

 so that the mixed nerves, while connected with the 

 nervous centre, react with the direct or inverse 

 current as the motor nerves do, and excite contrac- 

 tions on opening the direct current or closing the 

 inverse ; but the moment their connection with the 

 cord is destroyed they exhibit the phenomena 

 of mixed nerves, causing contraction with the 

 direct current on closing, and \\ith the inverse on 

 opening. 



