88 THE NKKVol'S SYSTK.M AM) ITS CONSIGNATION 



external conditions, though some of the factors deter- 

 mining it may lie far in the past. It is, of course. ;m act 

 of which one is conscious and which one desires to see 

 taking place. It is more profitable to attempt to unravel 

 the physical mechanism than to enter the metaphysical 

 discussion which is suggested. 



Movements of the muscles which are closely associated 

 with our consciousness and, as it seems to us, directed by 

 it are lii'ought to pass by neurons having their perikarya 

 in the cortex of the cerebrum. The fibers belonging to 



m 



Fig. 17. Hen- a "neuromuscular unit" like that in Fig. K\ is 

 sho\vn under the sway of a cortical motor cell ('0. The "projection 

 fiber" from this cell crosses the miilplnne of the body as suggested 

 by the line ///. In reality, a typic cortical cell probably influences a 

 number of cells of the lower order. 



such neurons never pursue ;m unbroken course to the 

 muscles which they can call into action, but run down to 

 form synapses with nerve-elements of the type already 

 mentioned, which ,-ire either near the place of origin of 

 those cranial nerves which contain motor fibers or else in 

 the spinal gray matter. The libers which transmit im- 

 pulses from the cortex to the neurons directly presiding 

 over the muscles belong to the class called projection 

 fibers. An odd peculiarity of these is that, as they descend, 

 they generally cm the midplane of the nervous system 



