484 THE NERVOUS SYSTEM. 



nerves, one is paralysed, the other may or may not be in- 

 adequate to maintain the sensibility of the entire part ; the 

 extent to which the sensibility is preserved corresponding 

 probably with the number of the fibres unaffected by the 

 paralysis. Thus when the ulnar nerve, which supplies the 

 fifth and a part of the fourth finger, is divided, the sensibility 

 of those parts is not preserved through the medium of the 

 branches which the ulnar derives from the median nerve ; 

 but the fourth and fifth fingers are permanently deprived 

 of sensibility. On the other hand, there are instances in 

 which the trunk of the chief sensitive nerve supplied to a 

 part having been divided, the sensibility of the part is 

 still preserved by inter-communicating fibres from a neigh- 

 bouring nerve-trunk. Thus, a case is related by Mr. 

 Savory in which, after excision of a portion of the mus- 

 culo-spiral nerve, the sensibility of some of the parts 

 supplied by it, although impaired, was not altogether lost, 

 probably on account of those fibres from the external 

 cutaneous nerve which are mingled with the radial branch 

 of the musculo-spiral. One of the uses of a nervous 

 plexus (p. 472) is here well illustrated. 



Several of the laws of action in motor nerves correspond 

 with the foregoing. Thus, the motor influence is propa- 

 gated only in the direction of the fibres going to the 

 muscles ; by irritation of a motor nerve, contractions are 

 excited in all the muscles supplied by the branches given 

 off by the nerve below the point irritated, and in those 

 muscles alone : the muscles supplied by the branches 

 which come off from the nerve at a higher point than 

 that irritated, are never directly excited to contraction. 

 No contraction, for instance, is produced in the frontal 

 muscle by irritating the branches of the facial nerve that 

 ramify upon the face ; because that muscle derives its 

 motor nerves from the trunk of the facial previous to these 

 branches. So, again, because the isolation of motor nerve- 

 fibres is as complete as that of sensitive ones, the irritation 

 of a part of the fibres of the motor nerve does not affect 



