SEGMENTS OF TRIGEMINAL NERVE-GROUP 279 
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 eappues 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, z.e. of the brain. 
Tue TRIGEMINAL NERVES AND THE MUSCLES SUPPLIED BY THEM. 
Turning now to the evidence as to the number of ventral seg- 
ments, 7.¢, 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 
