ELECTRICITY IN THE PHENOMENA OF ANIMAL LIFE. 443 



On the other band, it seems evident that it is in consequence not of 

 a static but of a dynamic action that the muscle as an organ maintains 

 its state of contraction. It behaves Hke a spring upon which a jet of 

 water constantly plays, and not like a spring loaded with a weight. 



In the former of these two cases the dynamic stress, constantly ke])t 

 up, is exerted against work, which involves expenditure of the moving 

 tiuid and, consequently, a certain amount of energy. In the same way 

 the sustained contraction of a muscle also demands a continued supply 

 of energy. 



The i)hysiological battery operates, then, to furnish this energy even 

 when tlte nuiscle is at rest, and as no external work is accomplished iu 

 this condition all the energy furnished is transformed into heat. 



It would be easy to multiply illustrations of this condition of special 

 equilibrium; we find it notably exhibited in the case of a ball sui)ported 

 on a jet of water and in tliat of a soaring bird, which contends with 

 the action of gravity by the rhythmic strokes of its wings. 



VI. — OF THK VAKIABILITY IN YIELD OF ENERGY BY THE AXIMAI. ORGANISM. 



It is easy to see that the quantity of energy consumed must increase 

 when the state of rest is followed by that of activity from whatever 

 cause, during which external and internal work are superadded to that 

 of the tonus existing during nuiscular repose. The reaction of the 

 organism is not constant; it varies in intensity with the condition of 

 sleep, of wakefulness, and of work. - - - 



It must be, then, of absolute necessity that the yield or output of 

 energy of the i)hysiological battery can regulate itself in due proportion, 

 so that there shall never l^e either excess or deficiency. 



Where shall we find the seat of this regulating action if it be not in 

 the nervous system itself, with the importance of which as anai)paratus 

 of stimulation we have already taken note? 



Assuming that the nervous system is not the producer of energy, but 

 that its function is only to distribute it, must we not hence conclude 

 that by means of its stimulating power it can modify, hasten, or retard 

 this distribution ? 



VII. — OF THE GENERAL REASON FOR THE EXISTENCE OF THE NERVOUS SYSTEM. 



We may picture to ourselves simply enough, in geneial princii^le, the 

 reason for the existence of the nervous system if we represent a nerve 

 as the main path by which electrical energy is carried from the point 

 at which it originates to a nerve center and from thence to an organ 

 of exi^enditure. 



In ijroportion as we ascend in the scale of matured animal forms or 

 in that of embryonic stages of development, and as we study more and 

 more perfect organisms, we observe the muscular and nervous systems 

 develop in extent and at tfie same time increase in the complexity of 

 their structure and functions. In i)roportion as the embryo undergoes 



