In the preceding paper (Buller, Eccles & Eccles, 1960) evidence was presented which suggested that the differentiation of slow muscles of the cat hind limb to a large extent failed to occur after certain operative procedures on the spinal cord. This finding indicates that in some way the central nervous system controls muscle differentiation. A more analytic investigation into this postulated influence of nerve on muscle has been accomplished by dividing and cross-uniting nerves to fast and slow muscles, so that motoneurones formerly innervating the fast muscle come to innervate the slow muscle by virtue of the regenerative outgrowth of their fibres, and vice versa for the motoneurones of the slow muscle. The effect of this crossed innervation on the speed of muscle contraction has been tested at varying times after the cross-union. These investigations have also been carried out on animals subjected to the operative procedures on the spinal cord, as previously described (Buller et al. 1960). Cross-union experiments allow in addition an investigation into the possible effects in the reverse direction, i.e. of speed of muscle contraction on the conduction velocity of the axons and the after-hyperpolarization of the motoneurones which innervate it. A preliminary account of part of this investigation has already been published (Bulier, . METHODSMost of the cross-union operations have been performed on kittens 2-3 weeks old, but some operations had a wide age dispersion: on kittens as young as 6 days; on kittens from 3 weeks to 4 months; and on full-grown cats at least 6 months old. The operations were performed on anaesthetized animals with full aseptic precautions. Usually the cross-union has been effected between the nerves to soleus and flexor digitorum longus muscles (FDL), but there have also been cross-union experiments between the nerves to peroneal muscles and soleus and between the nerves to gracilis and crureus. The position of the nerve suture has been as far distal as possible from the locus of the nerve cross and as far as possible

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Work in this field originates from Sherrington's observation (1909) that, when the limb of a decerebrate animal is flexed, there is at first a stretch reflex in the extensor muscle, but at a certain stage the extensor muscle concerned gives way and allows any degree of flexion to be imposed upon it. This was the lengthening reaction or clasp-knife effect which Sherrington described as an autogenetic proprioceptive reflex of inhibitory character. A first attempt at electrophysiological analysis was made by Denny-Brown (1928) who investigated the electromyogram during the tendon jerk, especially with reference to the silent period. He found that the silent period could be 'transmnitted from one extensor muscle to another' and observed that, in preparations in which this occurs readily, 'a stretch of one of these extensors, evoking in it a stretch reflex and then a lengthening reaction, causes partial or even complete inhibition of a stretch reflex present in another. ' Denny-Brown (1928) therefore associated it with the lengthening reaction.The next advance in this field came with the demonstration by Granit (1950) that contraction favours the appearance of the autogenetic inhibition as well as the associated inhibition of extensor muscles operating at neighbouring joints. This showed that receptors in series with the extrafusal fibres were responsible. McCouch, Deering & Stewart (1950) also made experiments with electrical stimulation of the quadriceps tendon and succeeded in obtaining a relatively pure early inhibition which they also ascribed to tendon endings. It was further demonstrated by Granit & Str6m (1951) that on stretching a muscle the onset of autogenetic inhibition had such a short latency that the responsible fibres must be in the group I range. Consequently it could be concluded that Golgi tendon afferents were responsible.These problems were approached in another way by Laporte & Lloyd (1952). They examined the conditioning effect which group I volleys from various-muscles exert on-monosynaptic reflexes. Inthis way it was established