410 Distribution of Fibres of Posterior Roots of Spinal Nerves. 



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 never- 

 theless, when its deep distribution is taken into account, to a com- 

 plete ray of tissue extending in an unbroken fashion from the median 

 plane of the body oat along the limb to (in the case of the nerves, 

 extending farthest into the limb) the very apex of it. This distribu- 

 tion conforms, therefore, with that shown in a previous paper to be 

 typical of the distribution of the ventral (motor) root. The distinc- 

 tion 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 paper 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 com- 

 parison of the distribution of the afferent and efferent roots in this 

 (Vlth 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 work ; the cross-lapping of the skin- 

 fields across the median line of the body, the overlapping of com- 

 ponent 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 over- 

 lapping of spinal ganglion-fields compared with that of peripheral 



