624 MANUAL OF PHYSIOLOGY. 



mitted to us ready for immediate application. Other paths re- 

 quired for the production of unusual combinations of movements 

 have to be worked out by the individual, and much of the diffi- 

 culty of learning 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 accurately coordinated motion. Indeed, the deli- 

 cacy 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 hereditary tendency to accomplish certain co- 

 ordinations required 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 irregularly diffused throughout all the cell territories (inde- 

 pendent of the ordinary paths they have been educated to fol- 

 low), and thus convulsive movements are excited in many parts 

 of the body. 



In like manner we can imagine that the unremitting activity 

 necessary to keep in check the impulses arriving from a constant 

 source of stimulation, such as intestinal worms, eventually fatigues 



