PRINCIPLES OF GENERAL PHYSIOLOGY 



below the patella, has a "synapse time" of 0'002 second, while that of the 

 flexion reflex is 0-004 second. If the latter involves two synapses and three 

 neurones, it appears that the knee-jerk only involves one synapse and two 

 neurones, an afferent one from the tendon, and the motor neurone to the 

 muscle fibre. The experiments were made by the use of the string galvano- 

 meter. By this means it is possible to record electrical changes in the afferent 

 fibres and in the efferent fibres, when the tendon is struck, and the various times 

 making up the total latency can be determined. The knee-jerk must, perhaps, 

 be regarded as an exceptional form of reflex in the higher vertebrates, although 

 it shows the possibility of an arc of two neurones only. The following measure- 

 ments from Jolly's paper are of interest : 



Subtracting the sum of the second, third, and fourth numbers from the total 

 latency, we have the time spent in passing synapses, as given in the last line. 



FIG. 146. DIAGRAM OF THE SPINAL ARCS INVOLVED IN THE SCRATCH REFLEX IN THE DOG. 



L, Receptive or afferent nerve path from the left foot. 



R, Receptive nerve path from the right foot. 



Ra, Rfi, Receptive nerve paths from hairs in different regions of the dorsal skin of the left side. 



/'(.'. The final common path, in this case the motor neurone to a flexor muscle of the hip. 



POL, P/3, Proprio-spinal neurones. 



(Sherrington, 1906, p. 119.) 



In actual experience, there is usually to be found at least one additional, 

 intermediate neurone, the whole of which is contained within the nerve centre. 

 Thus the " scratch reflex " of the dog consists of the following elements (Sherrington, 

 1906, p. 54), (Fig. 146). 



1. The receptor neurone from the skin of the back to the grey matter of a 

 certain spinal segment in the shoulder region. This forms a synapse in the grey 

 matter with 



2. A long neurone in the spinal cord itself (proprio-spinal), which passes 

 backwards to the segments of the hind leg. Here it forms a synapse with 



3. The motor neurone, whose axone supplies a flexor muscle of the leg 

 performing the scratching movement. 



There are thus three neurones and two synapses. There may, indeed, be more 



