588 SCIENCE PROGRESS 



is to affect the muscle must impinge. The motor nerve issuing 

 from that centre to the muscle is the sole channel by which 

 the condition of the motor centre can exert influence on the 

 muscle. The motor nerve consists of nerve-fibres, each of 

 which is a centrifugal thread from one of the motor nerve- 

 cells composing the discharging apparatus of the centre. What- 

 ever the seat and source of the various reflex actions which 

 excite or inhibit the muscle, each and all such reflexes take 

 effect upon it solely by affecting in the last instance the motor 

 cell and motor fibre, and by initiating or increasing or by 

 lessening or stopping a flow of nervous impulses down the 

 structures to the muscle. Hence in the chain of nervous con- 

 ductors which every reflex employs the motor cell and its fibre 

 constitute the last link and every reflex reaching the muscle 

 from whatever source must use them. For that reason the 

 motor cell and its centrifugal fibre is called the fi,nal common 

 path^ and the entrance or mouth to this final common path lies 

 in the motor centre. Indeed, the entrance or mouth of the 

 final common path is nothing else than the place of linkage 

 of the motor cell with the various afferent conductors which 

 impinge upon it. 



As to this place of linkage, the microscope shows that the 

 afferent conductors there impinging upon the motor cell 

 impinge upon it in two portions of its surface. The afferent 

 conductors themselves are delicate nerve-fibres, end-twigs of 

 stem-fibres from nerve-cells more or less distant. These end- 

 twigs terminate as seen in stained specimens specially treated 

 for. the microscope in tiny bulbous swellings set close to the 

 surface of the motor cell. They end both upon that part of 

 the motor cell which is the so-called cell-body and since it 

 contains the cell-nucleus is termed the perikaryon. They end 

 also upon the surface of the short, branched, tapering extensions 

 of the perikaryon, the so-called dendrites. With one part of 

 the motor cell the afferent terminals have no direct relation, 

 namely, with that long, thread-like extension from the peri- 

 karyon which constitutes the motor nerve-fibre and runs to 

 the muscle itself The meeting place of afferent conductor with 

 motor neurone is called the synapse and there the two though 

 not structurally continuous stand in functional connection. 

 There states of excitation induced in the afferent conductor on 

 reaching its terminals are transmitted across the synapse to 



