180 THYSIOLOGY OF MUSCLES AND NERVES. 



wb-ich cut the surfaces of the cylinder parallel to the 

 surfaces of its ends. The iso-electric surface repre- 

 senting a tension = 0, cuts the cylinder near its centre, 

 and divides it into two unequal halves, of which the 

 right is positive, and the left negative. The other iso- 

 electric curves cut the surfaces of the cylinder in par- 

 allel curved lines ; and the iso-electric curves repre- 

 senting the greatest positive and the greatest negative 

 tensions meet the surfaces at the central points of the 

 end surfaces of the cylinder which, in the figure given, 

 are marked + 6 and — h. 



The conditions are not always as simple as in this 

 case. If the body under examination is not a re- 

 gular cylinder, and if the electromotive force is not 

 situated exactly in its axis, then the arrangement of 

 the iso-electric surfaces is more complex. The body 

 under examination is, however, always occupied by a 

 system of current-planes inserted one within the other, 

 and a system of iso-electric surfaces can be constructed 

 which cut the outer surfaces of the body in curves of 

 one form or another. Along each curve of the outer 

 surface corresponding with an iso-electric surface the 

 same tension always prevails ; on two of these curves if 

 adjacent the tensions always differ. Regarding therefore 

 only the surface, it may be said that if an electro- 

 motive force is present within the body, this must cor- 

 respond with a definite arrangement of tensions on 

 the surface of the body. By studying this superficial 

 arrangement of the tensions we may therefore draw 

 conclusions from this as to the situation of the electro- 

 motive force within the body. 



3. The diverting vessels (fig. 38) above described 

 are not always sufficient for the purposes of research. 



