624 MANUAL OF PHYSIOLOGY. 



make beaten tracks, along which impulses can pass without hin- 

 drance. The nerve-paths along which the impulses, producing 

 certain movements commonly performed by every individual, have 

 to pass, are no doubt prepared by the long practice of our ances- 

 tors,- and the power of performing these actions is transmitted to 

 us ready for immediate application. Other paths required for the 

 production of unusual combinations of movements have to be 

 worked out by the individual, and much of the difficulty of learn- 

 ing any trade depends on the necessity of making impulses readily 

 traverse some definite directions, so as to excite certain groups of 

 cells to act synchronously and set certain sets of muscles in accu- 

 rately coordinated motion. Indeed, the delicacy of manipulation 

 required by some trades cannot be attained in the lifetime of one 

 individual ; thus, it is said to take three generations to make a 

 perfect glass-blower ; the grandson having the benefit of the he- 

 reditary tendency to accomplish certain coordinations acquired 

 by the life-long habit of the parents. 



The reflex convulsions that occur in poisoning with strychnia, 

 or as the result of some constant but slight stimulation, may be 

 explained as follows: 



We know that, besides the resistant thin paths of connection 

 between the cells of the cord, there also exist medullated fibres 

 short cuts, as it were for impulses to travel from one part of the 

 cord to another, for the various cell groups are in communication 

 with those situated in the other regions, by means of fibres that 

 lie in the white columns. Now, if we suppose the ordinary reflex 

 traffic of the cord-cells to be carried on without the assistance of 

 these direct lines of communication, we must assume that there is 

 some special means of shutting these fibres out of the working of 

 the reflex machine. Such special mechanisms do, in all proba- 

 bility, exist, and are in relationship with, or under the command 

 of the inhibitory cells of the higher centres. We may then sup- 

 pose that strychnia removes the power of these special agents, and 

 the impulses finding the direct ways from one part of the cord to 

 another open, take these routes, and are simultaneously and irregu- 

 larly diffused throughout all the cell territories (independent of 



