334 Dr. C. S. SherringtoD. On the Fibres of [Dec. 8, 



bouriug nerve-twigs, that are near their destination, is a necessity of 

 this arrangement. Thus, the dorsal collateral digital nerve, on the 

 tibial side of a digit, will resemble, in composition, the plantar 

 collateral digital on the tibial side of the same digit, although they 

 are derived from separate parent trunks. This is comparable to the 

 similarity of root composition exhibited by the several motor twigs 

 entering one and the same muscle. Thus, the tibialis anticus fre- 

 quently receives fibres from three motor spinal roots, and receives 

 those fibres by at least three several nerve-branches entering it ; the 

 root composition of each of the nerve-branches is approximately the 

 same. The dorsal digital supplying the cleft between the 1st and 

 2nd digits is interpolated in the series, of digitals from the mu.scu.lo- 

 cutaneous nerve, although it comes itself from the anterior tibial. 

 Yet it, by its root composition, falls into perfect series with the other 

 digital nerves. 



Each segmental skin-field spreads, to a certain extent, across neigh- 

 bouring segmental skin-fields. It has a fore-lap extending into 

 segmental fields immediately anterior to it ; and an after-lap extend- 

 ing into fields immediately posterior ; it has also cross-laps extending 

 into the corresponding fields on the other lateral half of the body, 

 both at the mid-dorsal line (the dorsal cross-lap) and the mid-ventral 

 line (the ventral cross-lap). The fore-lap and the after-lap are, 

 throughout the body, very great, and each region of skin appears to 

 be supplied by at least two sensory spinal roots. The over-lap of the 

 skin-fields of the individual filaments of a posterior root is great. 



The shape of a segmental skin-field is, where simplest, e.g., in the 

 trunk and neck, band-like, wrapping transversely round one lateral 

 half of the body ; it has fairly parallel edges, but is somewhat broader 

 near its ventral than at its dorsal end. In the limb the segmental 

 skin-fields are distorted from the simple band-like type. The distor- 

 tion of each segmental field in the hind-limb, and of some in the fore- 

 limb, is, in the full paper, analysed, and for each, the true anterior 

 border, the true posterior border, and the true dorsal and ventra 

 borders are found. This analysis is only possible after it has been 

 recognised that in the limb the cutaneous segments are not only dis- 

 torted but are seemingly dislocated from their attachments to the 

 mid-dorsal and mid-venbral lines of the trunk. 



The mid-dorsal line of the body may be said to, in the region of the 

 limb, extend outwards as a side branch, a secondary axis, almost at 

 right angles to itself. The same is done in the same region by the 

 mid-ventral line. Upon these dorsal and ventral side lines as upon 

 secondary dorsal and ventral axes, the cutaneous segments of the 

 limb are ranged, as though upon folded portions of the axial lines of 

 the trunk itself. 



The axial lines for the hind-limb slant outward from the trunk 



