86 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



We have little knowledge of the physiological activities of the end-brushes. 

 We know that much more time is lost in the central nervous processes than 

 would be required to transmit the excitation through nerve-fibres, and that the 

 time occupied is apparently the greater the longer the chain of nerve-cells en- 

 tering into the act. A part of this time is undoubtedly spent in the processes 

 occurring within the nerve-cells, but it is not unlikely that a portion of it may 

 be spent by the nerve end-brushes in the excitation of the cells. 



It is certain that the motor end-plates use up more time in the excitation of 

 the muscles than would be required for the transmission of the irritation 

 through a corresponding amount of nerve-substance. It is found by experi- 

 ment that a muscle does not contract so quickly if it be excited through its 

 nerve as when directly stimulated. Part of the lost time is spent in transmis- 

 sion of the excitation through the nerve, but after allowance has been made for 

 this loss there is a balance to be accounted for, and this is credited to the motor 

 end-plates. The average time used by the motor end-plate is found to be 

 0.0032 second. 1 There are many facts which go to show that the motor end- 

 organ is different physiologically from the nerve; viz. the latent period 

 of the motor end-plate, the effect of curare on the nerve end-plate as dis- 

 tinguished from nerve and muscle, the fact that the end-organ loses its vitality 

 quicker than do nerve and muscle when the blood-supply is cut off, and the 

 very existence of an end-organ distinguishable with the microscope. 



Conduction in Both Directions. (a) In Muscle. Wherever proto- 

 plasmic continuity exists, conductivity would seem to be possible ; moreover, 

 the active change excited by an irritant would seem to be able to pass in all 

 directions, though whether with the same facility is not known. Where the 

 spread of the excitatory process is accompanied by a change in form, as is the 

 case in many of the lower organisms and in muscle-tissue, it is not difficult to 

 trace the process. The rate at which the excitation spreads through the irrita- 

 ble substance is very rapid, and special arrangements have to be employed to 

 follow it, but the change is not so swift that its course cannot be accurately 

 determined. It has been found that if a muscle-fibre be stimulated, as nor- 

 mally, by a nerve-fibre, the active condition produced at the point of stimula- 

 tion spreads along the muscle-fibre in both directions to its extremities ; if the 

 fibre be artificially irritated at either end, the exciting change runs the length 

 of the fibre, regardless of the direction, and stimulates every part of it to con- 

 traction. 



(b) In Nerves. In the cases of nerves where excitation is accompanied by 

 no visible manifestation of activity, a definite answer to the question is not so 

 readily obtained. As long as a nerve is within the normal body, the activity 

 of the nerve-fibre can only be estimated from the response of the cell which the 

 nerve-fibre excites, and there is such an organ only at one extremity of the fibre. 

 Efforts have been made to elucidate the problem by attempting to unite the 

 central part of a cut sensory nerve with the peripheral part of a divided motor 

 nerve, and observing, after the healing was complete, whether excitation of the 

 1 Bernstein : Archiv filr Anatomie und Physiologic, 1882, p. 329. 



