220 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



optic nerves arise as processes of the neural tube, and are not 

 comparable to the remaining cranial nerves. Primarily each 

 segmental cranial nerve appears to have contained sensory and 

 motor fibres. The sensory fibres, like those of the spinal nerves, 



optic chias. 

 Seg. I. 

 Seg. II, 

 Seg. III. « 



Seg. IV. ' " ' 



Seg. V. 



3rd vent 



III (nerve) 

 IV: 



V. (sensory) 



4th vent. 

 V. (motor) 



. genie, gang. 

 IX. 



Seg. VII. 



Seg. VIII. 

 Seg. IX. 



Fig. 180.— A Diagram to show the Relationship of the Cranial Nerves to the Primitive 

 Segments of the Head. 



are developed from ganglionic cells derived from the neural crest, 

 and are of two kinds, somatic and visceral. The motor fibres 

 are developed from cells in the neural tube and are also 

 of two kinds, somatic and visceral. In the evolution of 

 the vertebrates there has been much reconstruction in the 

 arrangement of the segmental fibres, the sensory fibres of several 

 segments having become grouped together in the 5th nerve, 

 and the motor fibres of others in such nerves as the 10th 

 and 12th. 



The Segments to which the Cranial Nerves belong. 1st 

 Cranial Segment. — The motor nerve is the 3rd or oculo-motor. 



