ARRANGEMENT OF THE NERVOUS SYSTEM 571 



moreover, subsequent experiments have shown that the activity of 

 the ganghon cells varies directly with the quantity of oxygen supplied 

 to them. Hence, their power of oxidation can no longer be doubted. 



It has been established that prolonged muscular exercise gives 

 rise to fatigue substances, consisting of carbon dioxid, lactic acid and 

 monopotassium phosphate.^ In analogy with this observation, Dol- 

 ley^ recognizes a ''fatigue of depression *' throughout the body, which 

 results in consequence of the production of toxic substances, and a 

 "fatigue of excitation" which follows the excessive consumption of 

 nervous material. Thus, it is a common experience that excessive 

 muscular fatigue reduces our mental efficiency, while conversely, 

 mental fatigue weakens our muscular power and other bodily functions. 

 It is argued further that the highly organized centers are more suscep- 

 tible to fatigue than the ordinary reflex centers, because mental work 

 produces symptoms of fatigue with much greater ease than muscular 

 exercise. This is especially true of young children who ''go stale" 

 very quickly unless their mental training is properly balanced by rest 

 and play. But while we may feel justified in assuming that ganglion 

 cells give rise to fatigue substances, we have not succeeded as yet in 

 isolating these bodies, the only possible exception being carbon dioxid. 

 Winterstein^ has shown that the administration of this gas produces 

 an exhaustion of the nerve cells within a very short time. 



The Refractory Period of the Nerve Cell. — It will be recalled that 

 cardiac muscle is impervious to stimuli during systole but gradually 

 becomes more irritable as the end of the diastolic period is reached.^ 

 Systole is the period during which the contractile substance is used up, 

 and diastole the period during which it is again acquired. This 

 type of protoplasm, therefore, is not in a condition to receive stimuli 

 so long as those internal reactions are being promulgated which give 

 rise to its contraction. It again becomes receptive during its recuper- 

 ative period, i.e., during the diastole and pause. In a similar way, it 

 is held that nerve tissue undergoes cataboUc and anabolic changes, 

 and hence, a sufficient time must always be allowed to elapse between 

 two successive stimuli, otherwise the material will not be at hand with 

 which to produce the subsequent reaction. Thus, if the successive 

 stimuli are sent into nerve tissue with an increasing rapidity, a point 

 will eventually be reached when no reaction can result. The stimuli 

 then become ineffective, because not enough time has been allowed 

 for the renewal of that material which has been used up during the 



1 The formation of the so-called muscle toxins has been denied by Lee (Proc. 

 Soc. Exp. Med. and Biology, 1917). 



2 Intern. Monatsschrift fiir Anat. und Physiol., xxxi, 1914, 35. 



3 Zeitschr. fiir allg. Physiol., vi, 1906, 315. 



* Discovered by Marey, (Compt. rend., 1891) and applied to nerve tissue by 

 Broca and Richet (Compt. rend., 1897). These investigators found that the 

 cortical cells are unirritable for some time after the cessation of the muscular 

 spasms, such as occur in chorea and epilepsy. 



