Page:Proceedings of the Royal Society of London Vol 60.djvu/442

410 The Ilnd section of the communication deals with the degree of conformity between the distribution of the spinal ganglion fibres in the skin and their distribution in the underlying deep tissues of the limb. It is shown that, although the skin fields of the ganglia are in the middle of the limb region dislocated from the median line of the body, the distribution of the fibres of the root ganglion is nevertheless, when its deep distribution is taken into account, to a complete ray of tissue extending in an unbroken fashion from the median plane of the body out along the limb to (in the case of the nerves, extending farthest into the limb) the very apex of it. This distribution conforms, therefore, with that shown in a previous paper to be typical of the distribution of the ventral (motor) root. The distinction is not, therefore, as between afferent and efferent, but as between cutaneous and muscular. A detailed analysis of the distribution of the deep sensory fibres is in this pa,per carried out for the YIth lumbar spinal ganglion of Macacus rhesus; this ganglion was chosen because its skin-field, occupying the free apex of the lower limb, is one as far dislocated from the median line of the body as any in the whole spinal series, and presents, therefore, the greatest apparent discrepancy between the distribution of its afferent and efferent roots. A comparison of the distribution of the afferent and efferent roots in this (VIth lumbar) nerve was made by means of the Wallerian method ; the results show the peripheral distribution of the two to be minutely similar. From this, and from other observations given, the rule is put forward as a definitely established one that the sensory nerves of a skeletal muscle in all cases derive from the spinal ganglion (or ganglia) corresponding segmentally with that (or those) containing the motor cells, whence issue motor nerve-fibres to the muscle. The reflex arc, in which the afferent and efferent nerve-cells innervating a muscle are components, need not, therefore, as far as anatomical composition is concerned, involve irradiation through more than a single spinal segment.

Section III deals with general features of arrangement recognisable in the distribution of the roots; for instance, the determination of the position of the primary dorsal and ventral lines of the limbs, the examination of the asserted rotation of the limbs and of the asserted torsion of the limbs, and of the asserted homologies between muscles, &c., of the brachial and pelvic limbs respectively, by the criteria for re-examination of such questions provided by the facts elicited in the course of the woi’k ; the cross-lapping of the skinfields across the median line of the body, the overlapping of component parts of a single field, the serial overlapping of adjacent fields, the degree of overlapping in different regions of the body, the degree of overlapping in peripheral nerve-trunk fields, the amount of overlapping of spinal ganglion-fields compared with that of peripheral