GENERAL PHYSIOLOGY OF MUSCLE AND NEHVE. 97 



processes. The normal function of the nerve, however, the conduction of the 



nerve-impulse, seems to take place without any marked chemical change. 



Nature of the Conduction Process. — There have been a great many 

 views as to the nature of the conduction process, one after the other being 

 advanced and combated as physiological facts bearing on the question have 



been accumulated. It has been suggested that the whole nerve moved like a 

 bell-rope; that the nerve was a tube, and that a biting acid flowed along it ; 

 that the nerve contained an elastic fluid which was thrown into oscillations; 

 that it conducted an electric current, like a wire; that it was composed of 

 definitely arranged electro-motor molecules which exerted an electro-dynamic 

 effect on each other; that it was made up of chemical particles, which like 

 the particles of powder in a fuse, underwent an explosive change, each in 

 turn exciting its neighbor; that the irritant caused a chemical change, which 

 produced an alteration of the electrical condition of such a nature as to excite 

 neighboring parts to chemical change and thereby to electrical change, and 

 so alternating chemical and electrical changes progressed along the fibre in the 

 form of a wave; finally, that the molecules of the nerve-substance underwent 

 a form of physical vibration analogous to that assumed lor light. 



A discussion of these different theories, none of which can be regarded 

 as entirely satisfactory, cannot be entered upon here. 



Although the exact nature of the conduction process is not determined, 

 there seems little doubt that it is of the same type in all forms of protoplasm. 

 In all cases it is a property of the living substance of the cell and is lost 

 when the cell dies : the state of activity spreads like a wave in all directions 

 through the living substance, and is markedly altered by physical and chem- 

 ical influences which change the irritability of the living substance, and in 

 much the same way as this is altered ; continuity of protoplasm is absolutely 

 essential to conduction, hence the spread of the excitation change is limited 

 to the one cell, unless the cell is connected by protoplasmic bridges with 

 other cells, or possesses a specially differentiated exciting end-organ. 



In its details the conduction process exhibits many peculiarities in differ- 

 ent cells and even in the different parts of* the same cell. The receiving 

 organs at the extremities of the dendrites of different classes of neurones 

 differ widely in respect to structure, and in their capacity t<. react to different 

 kinds of stimuli and to transmit the state of excitation to the dendrite. The 

 exciting organs at the extremities of the axonee of different .'lasses of neurones 

 are of different types, and behave differently, the discharge of the exciting 

 process upon a muscle, gland, or nerve-cell being adjusted to the capacity for 

 reaction possessed by the organ in question. In each neurone the strands of 

 protoplasm which connect these distant receiving and exciting mechanisms 

 with the cell body, and the body of the cell itself, work each according to its 

 own nature. For example, the time spent by the phase of activity in tin- 

 body of a ganglion-cell of the posterior Bpinal root-ganglion, is far longer 

 than that used in a corresponding length of protoplasm in the dendrite of the 

 cell. Although the conduction process differs in its details even in different 



Vol. II.— 7 



