From what has been said concerning the more complex reflex actions, it is clear that the cells of the spinal cord are capable of arranging the discharge of nerve impulses, so as to bring about definite purposeful movements. This power of coordinating impulses, which is so striking in some reflex actions after the brain has been destroyed, is equally important in arranging efferent impulses and accomplishing ordinary voluntary movements. In fact, most of the details of the mode of working of the muscles are under the control of the cells of the spinal cord.
It will help us in formulating the mechanism if we suppose the resistance in the gray part of the cord to be much greater than that in the medullated nerve channels, and that throughout it the paths are so numerous that each individual nerve cell might be in communication with every other nerve cell. These paths are made passable by use; the oftener an impulse traverses a given route the more adapted such a route becomes for future traffic. Thus, by practice, we constantly freshen certain channels of intercommunication between the various cells of the cord and thus make beaten tracks, along which impulses can pass without hindrance. In a similar way certain groups of nerve cells acquire the habit of working together and exciting complex movements which at first were impossible. The nerve paths, along which the impulses, producing common movements, have to pass, are no doubt prepared by the long practice of our ancestors, and the power of performing these actions is transmitted to us ready for immediate application. Other paths connecting groups of cells required for the production of unusual combinations of movements have to be practiced by the individual, and much of the difficulty of learning any trade of special manual dexterity depends on the necessity of making impulses readily traverse definite directions, so as to excite certain groups of cells to act synchronously and set the required combination of muscles in accurately 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 require three generations to make a perfect glassblower; the grandson having the benefit of the hereditary tendency to accomplish certain coordinations acquired by the lifelong habit of the parents.
The importance of this technical education of the cells of the spinal cord in the execution of delicate manipulations will be felt if one attempt to imitate the movements of precision which a skilled craftsman executes without attention or voluntary effort even in the most careless exercise of his craft. The practice required for such education is experienced by any one who attains skill in the simplest special manipulation, from writing to playing the violin.