FUNCTIONS OF THE SPINAL COED. 793 



rapidly, from the acid than before ; such a mechanism is spoken of as 

 Setschenow's inhibitory centre. 



Reflex action may, likewise, be inhibited by stimulation of sensory 

 nerves. As an example of this may be mentioned the familiar experi- 

 ence of our ability to arrest a sneeze by compressing the skin of the nose 

 over the exit of the nasal nerve. 



Reflex action does not take place when strong electrical irritation is 

 applied to the trunk of a sensory nerve, but tetanus results; while, on the 

 other hand, a much weaker stimulation of the skin, either chemical or 

 mechanical, will readily produce reflex movement. This would, perhaps, 

 indicate that, together with the sensitive fibres, inhibitoiy nerves pass in 

 the trunks of the nerve to the spinal cord. 



The reflex functions of the spinal cord may be looked upon as one 

 of the preservative influences of the animal body, guarding all the inlets 

 and outlets of the economy. Through it the movements of respiration 

 are permitted to occur during the hours of sleep and waking. Let this 

 reflex action be lost in the medulla, and respiration ceases, the contents 

 of the rectum are involuntarily evacuated, the useful operation of wink- 

 ing, by which the conjunctiva is kept moist and the eye is protected, is 

 lost, and the acts of coughing and sneezing, so important for removing 

 foreign substances, would be alike impossible. 



The movements of the intestinal canal, although not entirely de- 

 pendent upon reflex action, are in a certain degree due to it. Many of 

 the phenomena which we consider as voluntary may be classed among 

 those which are reflex m their nature, as when, in walking, we may 

 unconsciously pass around an obstacle in our path, or unconsciously 

 perform many acts which are apparently purely voluntary in nature. 



As already mentioned, reflex actions are not solel} 7 motor in nature, 

 but may result in the production of changes in secretion, in the distribu- 

 tion of blood to a part, or in changes in nutrition. Illustrations of 

 excito-secretory phenomena are very numerous. 



If we touch the tongue with irritating or sapid substances the secre- 

 tion of saliva begins to flow through the instrumentality of the lingual 

 nerve ; the impression is conveyed to the medulla oblongata, whence an 

 efferent impulse is emitted through the chorda t3'inpani, as a result of 

 which the blood-vessels supplying the submaxillaiy gland are dilated and 

 an increased flow of saliva is produced. It has also been found that if 

 we stimulate the oral cavity the gastric secretion is poured out in large 

 quantities, indicating the action of condiments and spices in conditions 

 of feeble digestion. 



The reflex vaso-motor results are clearly evident as examples of 

 reflex action. If a sensory nerve is stimulated the tonic action of the 

 vaso-motor centre is increased, and the blood-vessels of the body are 



