GENERAL PHYSIOLOGY OF NERVE-CEXTBES. 785 



are found which are capable of regularly and rhythmically governing 

 complex movements, and, to that extent, it is automatic : the brain, 

 however, as the seat of mental activity, perception, volition, thought, 

 and memory, is the highest expression of the automatic functions of 

 nervous centres. The automatic functions of the cerebrum will sub- 

 sequently receive consideration in detail. 



3. INHIBITION. In a number of examples which were given as 

 illustrations of reflex action, as is well known, the will by its exertion 

 may prevent the appearance of the ordinary reflex result of the stimulus. 

 Thus, for example, touching the e3'eball tends to result in the pro- 

 duction of winking ; touching the throat, movements of vomiting and 

 coughing ; tickling the soles of the feet, contractions of the muscles of 

 the legs. All of these, as is well known, may be controlled by a volun- 

 tary impulse. 



One of the clearest illustrations of such inhibition of reflex action 

 and of its development by education is seen in the mechanism of defae- 

 cation. In the lower animals defaecation is a purety reflex action and, as 

 was described, results from the contact of the faecal mass with the 

 mucous membrane of the rectum. 



In infants, likewise, the same state of affairs occurs. By education 

 the will-power is capable of inhibiting the operations which result in 

 defaecation, or, in other words, checking the action of the nerve-cells 

 which control the co-ordinated movements in this process. 



A number of other illustrations might be given, of which, perhaps, 

 the clearest instance is seen in the action of the heart. 



If the pneumogastric nerve be stimulated in an animal in whom the 

 heart is beating in a normal manner with an interrupted current, the 

 heart is almost immediately slowed and may even be brought to a stand- 

 still ; such a result is explained by the statement that the pneumogas- 

 tric contains cardio-inhibitory fibres whose stimulation arrests the 

 automatic action of the motor ganglia of the heart. 



4. AUGMENTATION. In contradistinction to inhibition an afferent 

 impulse may increase action of nerve-centres. Thus, for example, the 

 vaso-motor centre located in the floor of the fourth ventricle controls 

 the calibre of the blood-vessels and keeps their walls in a state of con- 

 traction. The action of the vaso-motor centre may, however, be aug- 

 mented through various afferent impulses, the most striking of which is 

 seen in the great increase of blood pressure which follows stimulation 

 of sensory nerves in curarized animals. 



5. CO-ORDINATION. By this term is meant the power possessed b}' 

 the cells of the central nervous system of combining complex muscular 

 movements ordinarily of a reflex nature. Thus, for example, the act of 

 deglutition necessitates co-ordinate action of a large number of different 



50 



