304 INNEBVATION. [CHAP. xi. 



First, its anatomy in man. The origin by a double root denotes 

 a double function. Its peripheral distribution, however, gives more 

 valuable assistance. If distributed to muscles only, it clearly must 

 be motor: if to sentient surfaces only, sensitive and perhaps 

 excitor ; if to both, motor, and sentient, or excito-motor. 



Secondly, its anatomy in animals. The comparison of the origin 

 and distribution in the lower animals with those in man often 

 throws light on the function, by confirming the result of anatomical 

 investigation in the human subject, or by displaying either a pecu- 

 liar development of the nerve, in reference to some special function 

 proper to particular animals ; or, on the other hand, the noii-devc- 

 lopment of a nerve, or of a part of one, where some function may 

 be deficient. The enormous development of a branch of the fifth 

 nerve in animals with proboscides, or highly tactile snouts, of a 

 branch of the facial, where such an organ is very moveable, the 

 small size of the latter nerve where the muscles of the face are few, 

 are instances quite in point. 



Thirdly, experiment on animals just dead, or on those living. 

 The irritation of a motor nerve in an animal recently dead, causes 

 contraction of the muscles to which it is distributed. The section 

 of one in a living animal paralyses its muscles; but irritation of the 

 portion below the section causes contraction of those muscles which 

 that segment of the nerve supplies. The simplest way of applying 

 a stimulus for experimental purposes, is by passing a galvanic cur- 

 rent from a small battery. If the current be directed through a 

 nerve so that it shall pass along the smallest portion of it, by 

 placing one pole on one side of it, and the other on the opposite, 

 but a little lower down or higher up, we may gain a strong indica- 

 tion of the motor power of the nerve, if contractions are thereby 

 excited. This indication becomes certain if the same effect be 

 produced by galvanizing the nerve in this way, after it has been 

 separated from all connexion with the spinal cord or brain. Such 

 an experiment on a sensitive nerve would produce no motor effect. 

 Matteucci has shewn, that to produce the motor effect in a motor 

 nerve, the current must pass along some portion of the nerve-fibre, 

 however small ; and that a current directed precisely at right angles 

 to the fibres will not excite nervous power.* 



MM. Longet and Matteucci affirm, that a motor nerve may be 

 distinguished from a compound one by the different effect of open- 

 ing or closing an electric current on each under certain circum- 



* See an account of Matteucci's observations on the different effects of 

 electricity on nerves, in the appendix to this chapter, 



