606 NERVOUS SYSTEM. 



the nerve and are to be recognized, as well, in that portion through which the current 

 is passing ; but, between the poles, is a point where these conditions meet, as it were, 

 and where the excitability is unchanged. This has been called the neutral point. When 

 the galvanic current is of moderate strength, the neutral point is about half-way between 

 the poles. " When a weak current is used, the neutral point approaches the positive 

 pole, while in a strong current, it approaches the negative pole. In other words, in a 

 weak current the negative pole rules over a wider territory than the positive pole, 

 whereas in a strong current the positive pole prevails." (Rutherford.) 



Negative Variation. There remains to be considered one curious phenomenon, dis- 

 covered by Du Bois-Reyrnond, which depends upon the action of a rapidly-interrupted 

 current applied to an excitable nerve. If a galvanometer be applied to a living nerve so 

 as to indicate by its deviation the normal, or tranquil nerve-current, a rapidly-interrupted 

 current of electricity passed through a portion of the nerve, it is well known, produces a 

 tetanic condition of the muscles. If we now watch the needle of the galvanometer, it 

 will be observed to retrograde and will finally return to zero, indicating that the proper 

 nerve-current has been overcome. This will be observed to a slight degree under the 

 influence of mechanical or chemical stimulation of the nerve, the proper nerve-current 

 being diminished, but generally not abolished. This variation of the needle under the 

 influence of the tetanic condition has been called negative variation. We do not yet 

 know that it has any important physiological or pathological significance. 



CHAPTER XVIII. 



SPINAL NERVES-MOTOR CRANIAL NERVES. 



Special nerves coming from the spinal cord Cranial nerves Anatomical classification Physiological classification- 

 Motor oculi communis (third nerve) Physiological anatomy Properties and functions Influence upon the 

 movements of the iris Patheticus, or trochlearis (fourth nerve) Physiological anatomy Properties and func- 

 tions Motor oculi externus, or abducens (sixth nerve) Physiological anatomy Properties and functions 

 Motor nerves of the face Nerve of mastication (the small, or motor root of the fifth) Physiological anatomy 

 Deep origin Distribution Properties and functions of the nerve of mastication Facial nerve, or nerve of 

 expression (the portio dura of the seventh) Physiological anatomy Intermediary nerve of Wrisberg Decus- 

 sation of the fibres of origin of the facial Alternate paralysis Course and distribution of the facial Anasto- 

 moses with sensitive nerves Properties and functions of the facial Functions of the branches of the facial 

 within the aqueduct of Fallopius Functions of the chorda tympani Influence of various branches of the facial 

 upon the movements of the palate and uvula Functions of the external branches of the facial Spinal accessory 

 nerve (third division of the eighth) Physiological anatomy Properties and functions of the spinal accessory 

 Functions of the internal branch from the spinal accessory to the pneumogastric Influence of the spinal acces- 

 sory upon the heart Functions of the external, or muscular branch of the spinal accessory Sublingual, or 

 hypoglossal nerve (ninth) Physiological anatomy Properties and functions of the sublingual Glosso-labial 

 paralysis. 



Spinal Nerves. 



WITH a thorough knowledge of the general properties of the nerves belonging to the 

 cerebro-spinal system, the functions of most of the special nerves are apparent simply 

 from their anatomical relations. This is especially true of the spinal nerves. These, in 

 general terms, are distributed to the muscles of the trunk and extremities, to the sphinc- 

 ters and the integument covering these parts, the posterior segment of the head, and 

 a portion of the mucous membranes. It is evident, therefore, that an account of the 

 exact function of each nervous branch would necessitate a full description, not only of 

 the nerves, but of the muscles of the body, which is manifestly within the scope only of 

 elaborate treatises on descriptive anatomy. It is sufficient to indicate, in this connec- 

 tion, that there are thirty-one pairs of spinal nerves ; eight cervical, twelve dorsal, five 



