80 IRRITABILITY 



over the succeeding two or three days. This increase of the 

 nitrogen metaboHsm in its totality is not nearly as great as that 

 of the breaking down of nitrogen-free substances, but it is, 

 nevertheless, present and shows us that functional metabolism 

 cannot experience a lasting excitation without being followed by 

 secondary results in the entire cytoplastic metabolism. This 

 fact is even more strikingly illustrated in the alteration of the 

 entire volume of a living organism as produced by the lengthened 

 duration of functional stimulation. It has been long known, that 

 the muscle as the result of frequent functional excitation by 

 means of adequate nerve impulses, that is, prolonged activity, is 

 considerably increased in size, whereas in the absence of such 

 it loses more and more in volume. A hypertrophy of activity, 

 produced by functional stimuli, and the atrophy of inactivity, the 

 result of the discontinuance of the functional excitation, is uni- 

 versal and can be observed in the various tissues of our body. 

 We see it, for example, in the glands ; we see it in the skin and 

 we see it in the elements of the nervous system. Berger,^ for 

 instance, established the fact that the ganglion cells of the optic 

 lobe in the cerebrum of newborn dogs only reach their full 

 development when functionally excitated by adequate light stim- 

 uli (Figure 9, B), coming from the eye, whereas they remain in 

 the embryonic state when these light stimuli are eliminated. (Fig- 

 ure 9, A.) The cytoplastic increase of volume of the neurons 

 under the influence of functional stimuli is a fact of fundamental 

 importance for the entire happenings of the nervous system and 

 forms the physiological basis for reinforcement of reflexes, which, 

 in its turn, is essential for all acts of memory and intelligence. 

 For the increase in volume of the ganglion cell body is, when 

 functionally activated, accompanied at the same time by an 

 increase of specific capabilities and the intensity of discharge. 

 Its excitation impulses can, therefore, be conducted through a 

 greater number of neurons, with which it is connected, than would 

 be the case if development of the volume of the ganglion cell 

 increased to a less extent. 



1 Berger: "Experimentell-anatomische Studien uber die durch den Mangel optischer 

 Reize veranlassten Entwickelungschemmungen im Occipitallappen des Hundes und 

 der Katze." Arch. f. Psychiatrie, Bd. 33, 1909. 



