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 Piatt 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- 

 mentary 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 embryological 

 and anatomical facts ? 



Eemembering the tripartite 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 oculomotor ius, 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 



