SEGMENTS OF TRIGEMINAL NERVE-GROUP 267 
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. 
Further, as already mentioned, the eye-muscles in Ammoccetes 
must be considered by themselves; they do not belong in structure 
or position to the longitudinal somatic muscles innervated by the 
spinal nerves; their structure is not the same as that of the tubular 
constrictor or branchial muscles, but resembles that structure some- 
what ; their position is dorso-ventral rather than longitudinal ; they 
may be-looked upon as a primitive type of somatic muscles seg- 
mentally arranged, the direction of which was dorso-ventral. 
Anderson also has shown that the time of medullation of the 
nerves supplying these muscles is much earlier than that of the 
nerves belonging to the somatic trunk-muscles, their medullation 
taking place at the same time as that of the motor nerves supplying 
the striated visceral muscles; and Sherrington has observed that 
these muscles do not possess muscle-spindles, while all somatic 
trunk-muscles do, Both these observations are strong confirmation 
of the view that the eye-muscles must: be classified in a different 
category to the ordinary somatic trunk muscle group. 
What, then, is the interpretation of these various asa 
and anatomical facts ? 
Remembering thetripartite division of each segmental nerve-group 
in Limulus into (1) dorsal or sensory somatic nerve, (2) appendage- 
nerve, and (3) ventral somatic nerve, I venture to suggest that the 
three nerves—the oculomotorius, the trochlearis, and the abducens 
—represent the ventral somatic nerves of the prosoma, and partly 
also of the mesosoma ; that they are nerves, therefore, which may 
have originally contained sensory fibres, and which still contain the 
sensory fibres of the eye-muscles themselves, as stated by Sherrington. 
According to this suggestion, the eye-muscles are the sole survivors 
of the segmental dorso-ventral somatic muscles, so characteristic of 
the group from which I imagine the vertebrates to have sprung. In 
the mesosomatic region the dorso-ventral muscles which were retained 
were those of the appendages and not of the mesosoma itself, because 
the presumed ancestor breathed after the fashion of the water- 
breathing Limulus, by means of the dorso-ventral muscles of its 
