SEGMENTS OF TRIGEMINAL NERVE-GROUP 2 7 9 



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 Piatt 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 seg- 

 ments, 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, swp. rectus, 

 inf. rectus, int. rectus, inf. oblique, supplied by Illrd 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 



