THE PHENOMENA OF CONDUCTION IN NERVE 131 



stimulus at either point and the deflection of the string, the velocity 

 of the wave could easily be calculated. If stimulated in the axilla, 

 the deflection followed after an interval of 0.00578 second, and if 

 stimulated at the elbow, after 0.00442 second. As the distance be- 

 tween these two points amounts in most persons to 160-170 mm., 

 the wave must have progressed with a velocity of from 117 to 125 m. 

 in a second. 



Factors Altering the Speed of Conduction in Nerve. The funda- 

 mental condition for conduction is the anatomical continuity of the 

 nerve-fibers. If this has been broken in any way whatever, the excita- 

 tion must fail to reach the distant segment. An incomplete block 

 may be -established in various ways, for example, by compression, or 

 by crushing and stretching. Conduction then reappears gradually. 

 It may also be observed that the sensory fibers are somewhat less 

 resistant than the motor fibers. Thus, if pressure is brought to bear 

 upon the ulnar nerve at the elbow, the region supplied by it "goes to 

 sleep," but while this state is characterized by a simultaneous diminu- 

 tion of sensory and motor conduction, the former is usually depressed 

 in a much greater measure. Sensation, therefore, may be destroyed, 

 while the motor impulses are still able to pass through the block. 

 The return of conduction following the removal of the pressure is 

 usually associated with a peculiar pricking sensation in the region 

 supplied by this nerve. While no adequate explanation of this phe- 

 nomenon can be given, it is commonly assigned to processes of excita- 

 tion, i.e., to a temporary increase in the irritability of the nerve tissue 

 so affected. In fact, it has been stated by Weber, Schiff, and others 

 that an increased excitability of the nerve is also experienced directly 

 after its division. Compression-paralysis is usually ushered in by a 

 hyperactivity of the distant muscles. It seems, however, that the 

 development of this initial heightened irritability depends upon the 

 character of the injury as well as upon the quickness with which it is 

 effected. 



Mechanical influences are prone to give rise to an initial phase of 

 excitation unless permitted to act gradually, 1 while chemical agents 

 and cold do not. The degree of pressure which may be brought to 

 bear before conduction is abolished, has been determined by Ducc- 

 eschi 2 and Bethe. 3 The former employed a thin silk thread which 

 was drawn around the nerve and slightly weighted at one end. A 

 weight of a few grams sufficed to diminish the conduction, while 

 a reduction of the diameter of the nerve to one-third or one-fourth 

 of normal abolished it altogether. Naturally, a compression of this 

 intensity affects the enveloping sheaths and perifibrillar substance 



In this category belong the paralyses in the domain of the recurrent nerve 

 following aneurisms of the branches of the aorta, and the paralysis of the arm 

 muscles in consequence of the pressure of crutches. 



2 Pfliiger's Archiv, Ixxxiii, 1901, 38. 



3 Allg. Anat. und Physiol. des Nervensystemes, Leipzig, 1903. 



