62 INVERTEBRATE PHYSIOLOGY 



that there is yet another way by which the proprioceptors, at least in that 

 special situation, can act upon hitting performance. The superposition 

 effect indicates that the proprioceptive-center message is added to the optic- 

 center message (see Fig. 6c, dotted arrow). Yet, there is strong evidence 

 that the proprioceptive influence is smaller than the optic influence. Other- 

 wise, if it were equal or larger, the bias demonstrated in the head fastening 

 and in the loading experiments must be zero or even reversed in sign. 



(4) Finally the loading experiment gives a good proof of the biological 

 importance of the proprioceptive subcircuit, namely its ability to rule out 

 disturbing forces which act upon the neck motor system. According to 

 theory there should be no bias and no lowering of the hitting performance, 

 as long as these forces are eliminated completely. As soon as the head 

 begins to deviate from the intended position, say to the right, there would 

 occur a left bias — quite in accordance with the facts. It should be noted 

 that the subcircuit works not only against extraneous influences but also 

 against changes in the normal state of the muscles, caused for instance by 

 fatigue or any sort of metabolic disturbance. 



Second Series 



As the hypothesis put forward can be defined with mathematical pre- 

 cision, we should now make an attempt not merely to get knowledge about 

 the connections between the basic units of the system as we have done up 

 to now, but to determine quantitatively the functional relations predicted 

 by the theory and, finally, to calculate the constants of the system. I think it 

 is fairly clear what should be measured, namely the fixation-deficit in 

 normals and after bilateral nerve cutting. 



The plan of the procedure is simple enough ; the prothorax is fastened, 

 a fly is presented at a measurable direction a, and, if the animal faces it, the 

 position of the head /x is measured. 



I have used two different techniques, one of which is shown in Fig. 7. 

 The animal is sitting in its normal upside-down posture on a paper dial 

 hanging by threads of silk. The weight of the animal is compensated by 

 means of two pulleys and a counterweight not visible in the picture. A stick 

 of balsa wood gummed on the head projects through a hole, loose enough 

 to allow for rotation, and carries a pointer. Thus only the head movement 

 was limited to rotation about the relevant axis, all joints were free, and 

 the animal seemed to be comfortable even for weeks. In the second 

 apparatus also the head was completely free, bearing a small thin piece of 

 paper, which controlled the current of a photocell circuit. Finally, at the 

 output of the device, the position of the head, of the fly, and of the thorax 

 was registered automatically and continuously. 



It should be mentioned that this experimental series is not yet finished, 



