Also, the afferent or sensory nerves of the skin over the whole of this head-region are supplied by the trigeminal nerve, while the afferent nerves to the visceral surfaces are supplied by the vagus, glossopharyngeal and facial nerves.
In van Wijhe's original paper he arranged the segments belonging to the cranial nerves in the following table:—
| Segments. | Ventral nerve-roots and muscles derived from myotomes. | Visceral clefts | Dorsal nerve-roots and muscles. | ||||
|---|---|---|---|---|---|---|---|
| 1 | III. | M. rectus superior, m. rectus internus, m. rectus inferior, m. obliquus inferior | V. N. opthalmicus profundus | ||||
| 2 | IV. | M. obliquus superior | 1st Mandibular | V. | Masticating muscles. | ||
| 3 | VI. | M. rectus externus | VII.1 |
| |||
| 4 | — | — |
| VII.2 | |||
| 5 | — | — | 3rd 1st Branchial | IX. |
| ||
| 6 | — | — | 4th 2nd Brt" | X.1 | |||
| 7 | XII. |
| 5th 3rd Bra" | X.2 | |||
| 8 | XII. | 6th 4th Bra" | X.3 | ||||
| 9 | XII. | 7th 5th Bra" | X.4 | ||||
| Segments. | Ventral nerve-roots and muscles derived from myotomes. | Visceral clefts | ||||
|---|---|---|---|---|---|---|
| 1 | III. | M. rectus superior, m. rectus internus, m. rectus inferior, m. obliquus inferior | ||||
| 2 | IV. | M. obliquus superior | 1st Mandibular | |||
| 3 | VI. | M. rectus externus | ||||
| 4 | — | — |
| |||
| 5 | — | — | 3rd 1st Branchial | |||
| 6 | — | — | 4th 2nd Brt" | |||
| 7 | XII. |
| 5th 3rd Bra" | |||
| 8 | XII. | 6th 4th Bra" | ||||
| 9 | XII. | 7th 5th Bra" | ||||
| Segments. | Dorsal nerve-roots and muscles. | |||
|---|---|---|---|---|
| 1 | V. N. opthalmicus profundus | |||
| 2 | V. | Masticating muscles. | ||
| 3 | VII.1 |
| ||
| 4 | VII.2 | |||
| 5 | IX. |
| ||
| 6 | X.1 | |||
| 7 | X.2 | |||
| 8 | X.3 | |||
| 9 | X.4 | |||
As is seen in the table, van Wijhe attempts to arrange the cranial segmental nerves into dorsal and ventral roots, in accordance with the arrangement in the spinal region. In order to do this he calls the Vth, VIIth, IXth, and Xth nerves dorsal roots, although they are not purely sensory nerves, but contain motor fibres as well.
It is not accidental that he should have picked out for his dorsal roots the very nerves which form Charles Bell's lateral series of roots, inasmuch as this system of lateral roots, apart from dorsal and ventral roots, really is, as Charles Bell thought, an important separate system, dependent upon a separate segmentation in the embryo of the musculature supplied by these roots. This segmentation may receive the name of visceral or splanchnic in contradistinction to somatic, since all the muscles without exception belong to the visceral group of striated muscles.
These observations of van Wijhe lead directly to the following conclusion. In the cranial region there is evidence of a double set of segments, which may be called somatic and splanchnic. The somatic segments, consisting of the outer skin and the body musculature, are doubly innervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve. But the splanchnic segments are innervated by single roots, the vagus or Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth nerve, and trigeminal or Vth nerve, which are mixed, containing both sensory and motor fibres, thus differing markedly from the arrangement of the spinal nerves.
From this sketch it follows that the arrangement seen in the spinal cord, would result from the cranial arrangement if this third system of lateral roots were left out. Further, since the cranial system is the oldest, we must search in the invertebrate ancestor for a tripartite rather than a dual system of nerve-roots for each segment; a system composed of a dorsal root supplying only the sensory nerves of the skin-surfaces, a lateral mixed root supplying the system connected with respiration with both sensory and motor fibres, and a ventral root supplying the motor nerves to the body-musculature.
Comparison of the Appendage Nerves of Limulus and Branchipus to the Lateral Root System of the Vertebrate.