It is instructive to notice that the original position of attachment of this muscle is naturally posterior to that of the oculomotor group of muscles, and that Fürbringer, in his description of the eye-muscles of Petromyzon, asserts that this muscle in this primitive vertebrate form is not attached as in other vertebrates, but is posterior to the other muscles, so that he calls it the posterior rather than the superior oblique. The nature of the change by which the muscle known in the scorpion as the anterior dorso-plastron muscle (63) was probably converted into the superior oblique muscle of the vertebrate, is represented in the drawings Fig. [112], in which also are indicated the dwindling of the median eyes, and the progressive superiority of the lateral eyes, as well as the transformation of the recti muscle-group of the scorpion into the muscles supplied by the oculomotor nerve of the vertebrate.
With respect to the external rectus muscle, it follows naturally that if the muscles (64) and (65) are to follow suit with the rest of the group and become attached to the lateral eyes, they must take up an external position. These two muscles, which always run together, as seen in Fig. [110], A, the one belonging to the prosoma and the other to the mesosoma, are represented by the mandibular muscle of Miss Platt and the external rectus, the former derived from the walls of the last pro-otic head-cavity, the latter from the foremost of the opisthotic head-cavities.
Such, then, is the simple explanation of the origin of the eye-muscles which follows from my theory, and we see that the successive alterations of the position of the orbit, and, therefore, of the globe of the eye with its muscles, as we pass from Thyestes to man, is the natural consequence of the growth of the frontal bone, i.e. of the brain.
The Trigeminal Nerves and the Muscles supplied by them.
Turning now to the evidence as to the number of ventral segments, i.e. the motor and sensory supply to the prosomatic appendages afforded by the trigeminal nerve, we must, I think, come to the same conclusion as Dohrn, viz. that if there were originally seven dorsal or somatic segments in this region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles of the premandibular cavity, sup. rectus, inf. rectus, int. rectus, inf. oblique, supplied by IIIrd nerve; 6, 7, muscles of the mandibular cavity, sup. oblique, supplied by IVth nerve and muscle lost, there must have been also seven corresponding ventral or splanchnic segments supplied by the trigeminal. At present the evidence for such segments is nothing like so strong as for the corresponding somatic ones; there are, however, certain suggestive facts which point distinctly in this direction in connection with both the motor and sensory parts of the trigeminal. The origin of the trigeminal motor fibres in the central nervous system is most striking. We may take it for granted that a nucleus of cells giving origin to one or more segmental motor nerves will possess a greater or less longitudinal extension in the central nervous system, according to the number of fused separate segmental centres it represents. Thus a nucleus such as that of the IVth nerve or of the facial is small and compact in comparison to the extensive conjoint nucleus of the vagus and cranial accessory.
Upon examination of the motor nucleus of the trigeminal, we find a compact or well-defined nucleus, the nucl. masticatorius, the nerves of which supply the masseter, temporal, and other muscles, so that the anatomical evidence at first sight appears to bear out van Wijhe's conclusion that the motor trigeminal supplies at most two segments. Further examination, however, shows that this is not all, for the extraordinary so-called descending root of the Vth must be taken into consideration in any question of the origin of the motor elements, just as the equally striking ascending root enters into the consideration of the meaning of the sensory elements of the Vth.
It is not necessary here to discuss the controversy as to whether this descending root is motor or sensory. It is universally considered at present to be motor, and is believed to supply, as Kölliker suggested, among other muscles, the m. tensor tympani and the m. tensor veli palati. It is thus described by Obersteiner—
"From the region of the mid-brain the motor root receives an important addition of thick fibres, which form the cerebral or descending root. The large, round vesicular cells from which the fibres of the descending root arise form no single compact group, but are partly single, partly arranged like little bunches of grapes, as far as the region of the anterior corpora quadrigemina. The further we go brainwards, the smaller is the number of fibres. In the region of the anterior corpora quadrigemina, the few cells of origin are found more and more median; so that the uppermost trigeminal fibres descend in curves almost from the mid-line, as is shown by the exceptional occurrence of one or more of the characteristic cells above the aqueduct. At the height of the posterior commissure one finds the last of these trigeminal cells."
The anatomy of the Vth nerve reveals, then, three most striking facts:—
1. The motor nucleus of the Vth extends from the very commencement of the infra-infundibular region to nearly the commencement of the nucleus of the VIIth; in other words, the motor nucleus of the Vth extends through the whole prosomatic region, just as it must have done originally if its motor nerves supplied the muscles of the prosomatic appendages. Such an extended range of origin is indicative of the remains of an equally extended series of segmental centres or ganglia.