44« A MANUAL OF VETERINARY PHYSIOLOGY 



of the limb itself, and of all the other limbs as well. Its motor 

 path is a nerve common to air these. It is the final common 

 path, and a motor nerve to a muscle is a collection of such final 

 common paths. 



The afferent neurones comprising the private paths are several 

 times more numerous than the common paths ; in other words, 

 the outlet is much smaller than the inlet, and in consequence 

 there is competition for the right of way. In this matter there 

 is no compromise in a conflict, say, between two opposite 

 reflexes. One or the other must pass, both cannot. We shall 

 see this well illustrated in the ' scratch ' and ' foot reflex ' of 

 the dog. The victory in this struggle lies, as usual, on the side 

 of big battalions. The stronger the stimulus, the more likely is 

 it to occupy the road, especially if it be of a painful nature. The 

 stronger inhibits the weaker. The weaker is not necessarily 

 destroyed ; it may simply be held back, and permitted to follow 

 when the stronger rival has left the road open. Inhibition does 

 not take place in the motor nerve itself. The field of competi- 

 tion between the rival arcs seems to lie in the grey matter, 

 where the converging neurones come together at the commence- 

 ment of the common path, and here it is that some arcs drive 

 the final path into one kind of action ; others drive it into a 

 different kind of action ; and others, again, preclude it from 

 being activated by the rest. In studying the reflex act of stepping, 

 this feature will be well illustrated in the reciprocal innervation 

 of antagonistic muscles. We have previously learnt (p. 402) 

 that while the flexor muscles of a limb are contracting the tone 

 of the extensor muscles is inhibited ; this inhibition arises in the 

 nerve-centre, the stream of motor impulses along the motor 

 neurone being for the time cut off in the struggle for the common 

 path. This fact is of the utmost importance to a clear under- 

 standing of the question of muscular co-ordination, and further 

 consideration of the question will occur later. 



On the receptive surface the various impressions, such as light, 

 touch, heat, cold, and so forth, are picked out by the special 

 afferent nerves devoted to their transmission. The spinal con- 

 nection of different nerve-endings in the same area of skin is 

 assumed to be different, since stimuli suitable for one set of 

 movements are unsuitable for another. The scratch reflex needs 

 tickling or stimulation of a hair for its production. The reflex 

 of the ' extensor thrust,' which has yet to be spoken of, can only 

 be excited by pressing between the plantar cushion and the toe- 

 pads of the dog, and no other form of stimulation can invoke 

 it. A consideration of the special spinal reflexes now to be 

 described will prove object-lessons in the principles of the common 

 path, and the remarkable mechanisms it is capable of effecting. 



