Edinger says that such an array of clinical observations exists, and of facts derived from post-mortem dissections, that one may venture to designate the portion of the nucleus from which the innervation of each individual ocular muscle comes. He gives Starr's table, the latest of these numerous attempts, begun by Pick. According to Starr, the nuclei of the nerves to the individual muscles are arranged from before backward, thus—
| m. sphincter iridis. | m. ciliaris. |
| m. levator palpebræ. | m. rectus internus. |
| m. rectus superior. | m. rectus inferior. |
| m. obliquus inferior. |
Further, the evidence of the well-known physiological experiments of Hensen and Völckers that the terminal branches of the oculomotor nerve arise from a series of segments of the nucleus, arranged more or less one behind the other in a longitudinal row, leads them to the conclusion that the nuclei of origin are arranged as follows, proceeding from head to tail:—
| Nearest brain. | 1. | m. ciliaris. |
| 2. | m. sphincter iridis. | |
| 3. | m. rectus internus. | |
| 4. | m. rectus superior. | |
| 5. | m. levator palpebræ. | |
| 6. | m. rectus inferior. | |
| Most posterior. | 7. | m. obliquus inferior. |
It is instructive to compare this arrangement of Hensen and Völckers with the arrangement of the origin of these muscles from the premandibular cavity as given by Miss Platt.
Thus she states that the most posterior part of the premandibular cavity is cut off so as to form a separate cavity, resembling, except in position, the anterior cavity; this separate, most posterior part gives origin to the inferior oblique muscle. She then goes on to describe how the dorsal wall of the remainder of the premandibular cavity becomes thickened, to form posteriorly the rudiment of the inferior rectus and anteriorly the rudiments of the superior and internal recti, a slight depression in the wall of the cavity separating these rudiments. The internal rectus is the more median of the two anterior muscles. In other words, her evidence points not only to a fusion of somites to form the premandibular cavity, but also to the arrangement of these somites as follows, from head to tail: (1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior oblique—an order precisely the same as that of Hensen and Völckers, and of Starr.
I conclude, from the agreement between the anatomical, physiological, and morphological evidence, that the IIIrd and IVth nerves contain the motor somatic nerves belonging to the same segments as the motor trigeminal, in other words, to the prosomatic segments, so that the eye-muscles, innervated by III. and IV., represent segmental muscles belonging to the prosoma. Further, I conclude that originally there were seven prosomatic segments, the first of which is represented by the anterior cavity described by Miss Platt, and does not form any permanent muscles; that the next four belong to the premandibular cavity, and the muscles formed are the superior rectus, internal rectus, inferior rectus, and inferior oblique; and that the last two belong to the mandibular cavity, the muscles formed being Miss Platt's mandibular muscle and the superior oblique. It is, to say the least of it, a striking coincidence that such an arrangement of the cœlomic cavities as here given should be so closely mimicked by the arrangement in the prosomatic region of Limulus as already mentioned; it suggests inevitably that the head-cavities of the vertebrate are nothing more than the prosomatic and mesosomatic segmental cœlomic cavities, as found in animals such as Limulus. In the table on p. [253], I have inserted the segments in the vertebrate for comparison with those of Limulus.
Before we can come to any conclusion as to the original position of these eye-muscles, it is necessary to consider the VIth nerve and the external rectus muscle. This nerve and this muscle belong to van Wijhe's 4th segment. The muscle is, therefore, the somatic segmental muscle belonging to the same segment as the facial and is, in fact, a segmental muscle belonging not to the prosoma, but to the mesosoma. Neal comes to the conclusion that the existing abducens is the only root which remains permanent among a whole series of corresponding ventral roots belonging to the opisthotic segments, and further points out that the external rectus was originally an opisthotic muscle which has taken up a pro-otic position, or, translating this statement into the language of Limulus, etc., it is a mesosomatic muscle which has taken up a prosomatic position.
There is, however, another muscle—the Retractor oculi—belonging to the same group which is innervated by the VIth nerve. Quite recently Edgeworth has shown that in birds and reptiles this muscle belongs to the hyoid segment; so that in this respect also the hyoid segment proclaims its double nature.
With respect to the external rectus muscle, Miss Platt has shown that the mandibular muscle is formed close alongside the external rectus, so that the two are in close relationship as long as the former exists.