70 



VETERINARY PHYSIOLOGY 



and the further the point stimulated is from the structure 

 acted upon, the longer is this latent period. This indicates 

 that the change does not develop simultaneously throughout 

 the neuron, but, starting from one point, is conducted along it. 

 (a) The rate of conduction may be determined — 

 1st By stimulating a nerve going to a muscle at two 

 points at a known distance from one another, and measuring 

 the difference of time which elapses between the contraction 



»*-^ 



Fig. 30. — M, Muscle attached to crank lever marking on revolving drum. The 

 secondary circuit of an induction coil is connected with a commutator, 

 with the crossed wires removed so that the current may be sent either 

 through the wires going to the nerve at A far from the muscle, or at B, 

 a point at a measured distance nearer the muscle. On the drum, A 

 represents the onset of contraction on stimulating at ^, and B the onset 

 on stimulating at B. To secure stimulation in each case with the drum 

 in the same position, the make and break of the primary circuit is 

 caused by the point of K touching and quitting the point P. 



resulting from stimulation at each (fig. 30) (Practical 

 Physiology). 



2nd. By taking advantage of the fact that the conducting 

 part of a neuron is electro-positive, i.e. like the zinc element in 

 a galvanic cell, to the rest (p. 60), and by finding how long 

 after stimulation at one point this electric change reaches 

 another point at a measured distance from it (fig. 81). 



The rate of conduction varies considerably; everything 

 stimulating protoplasmic activity accelerating, and everything 



