THE PHYSIOLOGY OF MUSCLE 493 



These experiments demonstrate the injury-current of the tissues and 

 the great irritability of nerve as well. The term demarcation-current 

 signifies that the injury-current originates at the dividing-line between 

 the normal and the injured tissue. It is also sometimes ill-called the 

 current of rest. The direction of the demarcation-current is always from 

 a normal point round through a connecting conductor to an injured 

 point: the latter is "negative" to the former. Theories as to the cause 

 of the demarcation-, rest-, or injury-current have been various, one 

 sort "molecular" and the other chemical. Du Bois Reymond originated 

 the former, and especially Hermann and Hering have elaborated the 

 latter hypothesis. This chemical theory supposes that the katabolic 

 changes consequent on injury reduce the electromotive force of the part 

 making it less than (negative to) that of the normal region. These 

 theories apply to nerve as well as to muscle. 



The strength of this current is much smaller in nerve than in muscle; 

 in the former its strength is from 0.005 to 0.030 volt. In muscle it may 

 reach 0.1 volt. In nerve, owing to the relatively quick death of the 

 tissues, it rapidly disappears. Dead protoplasm gives no current of any 

 sort, nor do normal tissues so far as we know except when functioning. 

 It is then only in necrobiotic protoplasm that this sort of electricity is 

 set free. Its significance is not as yet clear, but its theoretical importance 

 is obvious. 



Expt. 52. Direction of Current Affects Contraction. (Apparatus: 

 Wax, sartorius muscle, rheocord.) Make a wax trough just large 

 enough to contain the sartorious muscle, and place the moistened muscle 

 within it. Place wires from the rheocord (block and anode) on either 

 end of the trough and close the key. The muscle contracts. Place the 

 wires on opposite sides of trough so that current goes exactly across the 

 muscle at right angles. The muscle does not contract. At any other 

 angle than this the current is an effective stimulus. Use various strengths 

 of the electricity. 



It is not easy to account for this phenomenon definitely, but there are 

 two hypotheses. One is that the lack of response is due to the balancing 

 of the two opposed influences at the cathode and anode, leaving the 

 resultant zero. This could occur only when the current was exactly 

 transverse, hence the difficulty of a good result. At 45 angle the result 

 is less than when the direction of the current is along the fibers. The 

 second "explanation" lies in the much greater resistance shown to 

 electricity's passage across fibers than with them. In nerve, for example, 

 where the same phenomena obtain, the longitudinal resistance is said to 

 be 2,500,000 times the resistance of a like length of mercury, while the 

 transverse resistance of nerve is 12,500,000 times greater. In these cases 

 the electricity might not pass through the excitable tissues at all. 



Expt. 53. Unipolar Induction. (Apparatus: Inductorium, four 

 nerve-muscle preparations.) (A) Make four nerve-muscle preparations 

 and place them on a perfectly dry glass plate so that the nerve of each 

 shall be over the muscle of the next except the last. Push the secondary 



