84 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in contact with the bodies and dendrites of the cells to be excited, and whether 

 this relation would be sufficiently close for a transmission of excitation, or 

 whether they should be considered as specially differentiated exciting mechan- 

 isms, which do not simply transmit the condition of excitation by a proc- 

 ess of conduction, but which develop a special form of physiological stimulus, 

 and through this excite the second neurone to activity. 



Of late, certain histologists claim to have traced the fibrillae of the axone 

 of one neurone into the cell-body of another neurone, and have even suggested 

 that the nerve impulse from the first might be transmitted through the cell- 

 body of the second and into its branches without the intervention of the pro- 

 toplasm of the body of the cell. 



It is possible that in some eases the axone of the exciting neurone may, 

 instead of ending close to the neurone to be excited, penetrate it and end in 

 its substance, just as the motor end-organ penetrates into the sarcoplasma 

 of the muscle-fibre. This could happen, and yet the protoplasms of the two 

 cells might preserve" their individuality. 



There are many facts which show that, physiologically at least, the two 

 neurones act as wholly independent mechanisms. These will be dealt with 

 more at length in the section devoted to the physiology of the central nervous 

 system. Suffice it to say, the end-brush at the extremity of the axone can 

 excite the cell body of another neurone, but cannot be excited by it. A 

 reflex act involving only two neurones requires more time than could be 

 used in simple conduction through the two cells. The character of the 

 impulse sent out of the spinal cord by the efferent cell may be very different 

 from that passing in along the afferent cell — e. g., the efferent impulse may be 

 stronger than the afferent ; the strength of efferent discharge may vary greatly 

 within short intervals of time even when the strength of the afferent impulses 

 remains the same ; Aveak afferent impulses may. by summation, lead to a 

 strong efferent discharge, and continuous afferent stimulation may awaken 

 rhythmic efferent discharges. 



In short, phvsiological facts are all opposed to the idea that there is con- 

 tinuity of protoplasm of different nerve-cells, and in favor of the view that the 

 end-brush, like the motor end-plate, acts as a specialized exciting mechanism. 



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 



