EXAMINATION OF SOME SPINAL NERVES 6i 
does not actually attain the mid-dorsal line anywhere, although it approaches on the scalp at one 
point somewhat closely (see page 60). It is therefore very unlikely that the field is of complete 
segmental character. 
A third feature in which the separate skin-fields of the three divisions of the trigeminus 
differ from the fields of complete segmental (spinal) nerves is the following : The overlapping of 
fields of contiguous spinal nerves is regular and very considerable ; the contiguous skin-fields may 
have half of their respective areas in common. With the fields of the three divisions of the 
trigeminus the extent of mutual overlap is irregular and not nearly so great. Thus, on the 
conjunctiva the fields of the ist and 2nd divisions meet, but hardly overlap at all ,• similarly 
those of the 2nd and 3rd divisions at the angle of the mouth. I do not know of detailed 
observations on the mutual overlap of the skin-fields of peripheral nerves, but in respect of their 
mutual overlap the skin-fields of the cranial Vth are very different from the skin-fields of complete 
segmental (spinal) nerves, and the skin-fields of peripheral nerve-trunks, as tar as I have myself 
investigated them, resemble in this feature the divisions of the trigeminus (see below, page 96). 
Analysis of the sensory portion of the cranial Vth nerve, therefore, by this method fails to 
resolve it from a single segmental nerve into a series. 
The fact that the fibres of the sensory root of the trigeminus, namely, the fibres running 
proximally from the Gasserian ganglion, have a long region of origin (from the mid-brain in 
front to the level of the Ilnd cervical nerve behind), also fails as an argument in proof that the 
nerve belongs to a immber of metameres. The fibres from the root ganglion of a spinal nerve 
grow into the central nervous axis, and there penetrate to various distances, becoming connected 
with not one segment, but with many ; thus, those of the sacral ganglia reach even to brachial 
and to bulbar segments. As regards the so-called surface origin of the sensory spinal root, it is not 
very uncommon to find filaments of the hinder of two nerve-roots crossing filaments from the root 
next in front, demonstrating to the naked eye the insufficiency of the surface origin of the root to 
serve as criterion of segmental position, cf. infra, P- 137 
It is the position of the ganglion, and not that of the so-called spinal ' origin,' that is the 
key to the segmental position. In other words, the situation of the nerve-cells, not that of the nerve- 
fibres, marks the segmental position of the sensory nerve. Growing out from the ganglion-cells, its fibres 
wander distally into skin and muscles, and these trespass, as I have pointed out, considerably 
beyond the limits of their proper metamere, for they considerably overlap. Centripetally they 
extend even more boldly into regions outside their original metamere, in fact, into segments quite 
remote. A similar arrangement exists in motor nerves, but I* and A. S. F. GRUNBAUMt have 
pointed out that the region of spinal origin of the fibres of a motor root, unlike that of the 
sensory root, is of strictly local extent in the spinal cord, confined to one, its own, segment. 
That is to say, the seat of the motor cells of the segmental nerve is, like that of the sensory cells 
of the segmental nerve, not plurisegmental, but confined to one, the original, segment. The 
nerve-cells of a metamere do not wander beyond the confines of the metamere in which they 
* 'Journal of Physiology,' vol. 13. 
■\ Ibid., vol. 15^ 
