120 PHYSIOLOGY OF MUSCLES AND NERVES. 



bility at the various points of the nerve is consecutively 

 determined in the way described above, it is generally 

 found that the excitability of the upper part of the 

 nerve is greater than that of the lower. There is, how- 

 ever, no great regularity in this character. Sometimes a 

 point is found in the centre of the nerve which is less 

 irritable than those immediately above and below it. 

 Very frequently the most excitable point occurs, not 

 immediately at the cut end, but at some little distance 

 from this ; so that, on proceeding downward, it is found 

 to increase at first, and then, at a yet lower point, to 

 decrease again. If such a nerve is observed for some 

 little time, its excitability at the various points being 

 tested every five minutes, it is found that the excita- 

 bility alters especially soon at the upper end ; it de- 

 creases, and in a short time is entirely extinguished, so 

 that no muscular pulsations can afterwards be elicited 

 from the upper parts even by the most powerful 

 currents. The nerve is then said to be dead in its 

 upper parts, and this death proceeds gradually down- 

 Avard in the nerve, so that pulsations can only be 

 obtained by irritating the part situated nearest the 

 muscle, and at a little later period even this part 

 becomes dead. After the whole nerve is dead, pul- 

 sations may yet always be obtained for a time by 

 direct irritation of the muscle. The muscle does not 

 usually die until much later than the nerve Yet in a 

 quite fresh preparation of the nerve and muscle, the 

 latter is always less excitable than the former, and 

 a much stronger irritant is required to excite the 

 muscle directly, than indirectly through the nerve. 

 In all these experiments the nerve must be care- 

 fully protected from drying up, as otherwise its excita- 



