86 THE NERVOUS SYSTEM 



The brain can exert normally almost any degree of inhibitory control 

 over the reflex actions. It commonly acts in this way provided it has been 

 made aware that the reflex influences are to pass, and provided also that 

 the neuro-muscular mechanism has not passed by long-inherited habit 

 into a condition of practical atomatism such as is observed, for example, 

 in the heart and intestines. Inhibition is one of the great problems of the 

 day in physiology; about its cause, its actual mode of working, and its 

 limits of influence we still know but very little. 



COORDINATION. Coordination of movements is accomplished through 

 the meeting of the separate nerve-paths in cells, in plexuses, or in the 

 "centers" of the central nervous system. According to the neurone- 

 theory, each fiber (axone) being the unit of conduction, coordination can 

 scarcely be supposed to take place 'within the cell, but rather in the 

 groups of neurones called centers. Various kinds of centers have been 

 already described. If, however, we consider the fibril within the 

 fiber as the conducting unit, we might suppose that the various 

 impulses coming in over the hundreds of fibrils may be arranged and 

 connected within the body of the cell, the better to serve the various 

 actions of muscles, the secretion of glands, the functions of the sense- 

 organs, or other functions. We cannot as yet decide between these 

 opposed points of view of the nervous system because of the histological 

 uncertainty. 



In its function of unifying the different actions of the body, the 

 coordinating process is essential, as is readily seen from any one of 

 numerous complex muscular acts. Take, for example, speech. (See 

 page 394.) This complicated process in its mechanical aspects is essen- 

 tially produced by draughts of air to and from the lungs which cause 

 vibration of the vocal cords, while the throat, tongue, lips, teeth, etc., 

 are adapted at the same time to the requirements of word -enunciation. 

 To produce the requisite draughts of air the whole respiratory muscular- 

 mechansim has to act in an exactly suitable way, inspiration and expira- 

 tion being carefully adapted in force, frequency, and continuance to 

 the exact needs. This means not only numerous afferent impulses 

 passing continually to the speech-center in the temporal lobe of the 

 brain, but hundreds and probably thousands of efferent impulses passing 

 thence indirectly (by way of the respiratory center) to the numerous 

 muscles of respiration. None of these cross-striated muscles and no 

 portion of any will work by itself, but every part must be directed in 

 exactly the proper way to produce the precise movements demanded of 

 them. Similarly in the larynx, numerous small muscles must be made 

 to act together as they have acted in producing similar sounds since 

 the individual learned to talk. This mechanism is by itself one of the 

 most complex of muscular coordinations. The soft palate must be 

 innervated in a certain way, and the tongue must be made to select 

 out of its multitude of movements just those, and none others, required 

 for the particular words expressed. The lips take part also and involve 

 exactly the right innervation of many small muscles, while the muscles 



