148 THEORETICAL BIOLOGY 



system as a system of tubes in which alterations of pressure 

 and quantity are necessary. 



This tube system is especially closely linked up with the 

 muscles ; these are roused into activity by the waves of 

 excitation in the nervous system, and the muscular excitation 

 reacts again on the nervous. For this purpose, each muscle- 

 fibre is connected with a motor-centre, which we may imagine 

 as a self-working hydraulic apparatus, regulating both the 

 pressure and the quantity, and throughout faithfully reflect- 

 ing the condition of excitation of the muscle obeying it. I 

 have called these centres representatives, because their duty is 

 to represent the muscles in the nervous system. 



We find the effector connections between nervous system 

 and muscles already perfectly developed in the lowest animals, 

 for even these, if they are to live, must have perfect control over 

 the movements they perform, few and simple though these be. 



In the lower animals we find the representatives grouped 

 round a simple nerve network, which has so-called tone- 

 centres ; these tone-centres are likewise automatic hydraulic 

 apparatuses, controlling the state of excitation of the muscula- 

 ture as a whole. Into this central network flow the excitations 

 coming from the receptors. 



If the central network is not subdivided, they all have 

 the same effect on the representatives of the muscles, which, 

 according to a very simple law, are set in vibration. The 

 excitation always flows along in the direction of the extended 

 muscle, and, since most muscles are constructed as pairs of 

 opposites, which reciprocally extend one another, the con- 

 traction of the one muscle produces the extension of its 

 opposite, and so opens, as it were, the gateway for the excita- 

 tion to enter. And thus a forward movement follows on 

 every stimulus. 



As we ascend the ladder of the animal kingdom, we notice 

 that, first of aU, differentiation sets in in the motor apparatus. 



