580 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



E. REFLEX RESPONSES TO DIFFERENT STIMULI 



The kind of stimulus employed has much to do with the appearance of 

 reflexes. Not only the end organs of the nerves of special sense, but also 

 the nerves of the internal organs (cf. pages 264, 270) are adapted to receive 

 stimuli of certain special kinds. 



Since the researches of Marshall Hall, we have known that in general a 

 reflex is more easily discharged by stimulation of the peripheral end organ 

 than by stimulation of the corresponding afferent nerve trunk. The cause 

 of this difference probably is, that peripheral end organs just because they 

 are adapted to receive stimuli of a special kind react to a stimulus of a definite 

 intensity more powerfully than does the nerve trunk. For this reason arti- 

 ficial stimulation of an afferent nerve trunk never gives a complete repro- 

 duction of the reflex functional capacity of the nervous system. 



We are compelled for want of space to pass over the observations which have 

 been made with regard to the different effects of mechanical, chemical, thermal 

 and electrical stimulation of the same efferent nerves. 



6. AUTOMATIC EXCITATION 



It is impossible at present to give an accurate estimate of the importance 

 of automatic excitation, either in the central nervous system or in peripheral 

 organs. The ease with which reflex effects can be ascertained has probably 

 been the occasion of some neglect of this question. 



That this form of excitation is extremely important, however, requires no 

 demonstration. By special effort a person can hold his breath, say, anywhere 

 from thirty seconds to several minutes; but he cannot, even with the utmost 

 power of his will, voluntarily stop respiration altogether. This overpowering 

 excitation of the respiratory center is the work of accumulated decomposition 

 products in the blood or lymph. We have seen that the breath volume is 

 increased by muscular work. This again is due primarily to the stimulating 

 effect of the decomposition products on the respiratory center, although the 

 stimulation of afferent nerve fibers may play some part also. 



The attendant effects of asphyxiation upon the circulatory system and 

 the skeletal muscles are a witness of how other centers in the brain and spinal 

 cord may be thrown into a state of extreme activity by decomposition products 

 present in excess. 



Since we have good reasons for thinking that the respiratory center is 

 stimulated normally by products of metabolism, albeit its activity is often 

 regulated by reflexes, we may suppose that automatic excitation plays a con- 

 siderable part in the tonic stimulation of other nerve centers, and that in 

 general this is the inciting agency behind the coarser functions of many 

 organs, whereas their finer adjustment to the momentary needs is accomplished 

 through the various reflexes which play upon the corresponding centers. 



