THE BRAIN AND SPINAL CORD. 219 



myelinated about the 5th and 6th months after birth. As 

 the processes grow downwards in the lateral wall of the cerebral 

 vesicle they pierce the corpus striatum, dividing it into the 

 caudate and lenticular nuclei. 



The Secondary Sulci, which divide the superior and middle 

 frontal convolutions, the calloso-marginal, the parietal and occi- 

 pital gyri, appear in the 9 th month. They are for the greater 

 part peculiar to the human brain. 



THE CRANIAL NERVES. 



The differentiation of the simple neural tube of the embryo 

 has thus far been followed into the complicated central nervous 

 system of the adult. It is now necessary to make a short survey 

 of the arrangement of the cranial nerves and see what evidence 

 they afford of a segmental arrangement of the cephalic part of 

 the neural tube. 



The Cranial Nerves. — The segmental arrangement of the 

 nerves of the body has been already discussed (page 158). Even 

 although the human trunk is highly specialized the 33 or more 

 segments of which it is made up can still be recognised from 

 the arrangement of the spinal nerves ; each segment is constituted 

 on a similar principle, and it becomes increasingly difficult to 

 deny that man and the whole kingdom of vertebrates are derived 

 from a form in which all the segments of the body were 

 identical. 



The head has become even more highly specialized than the 

 trunk, and in it evidence of segmentation is accordingly more 

 difficult to detect. Most of the evidence at present at our 

 disposal indicates the presence of nine segments in the head. 

 That is to say that the mammalian head is the derivative 

 of a structure which was made up of nine segments, every 

 one of which was originally constituted very much alike. 

 Each had a similar arrangement of nerves and muscles, a similar 

 arrangement of vessels, and provided with a similar pair of 

 appendages. 



In Eig. 180 is diagrammatised the relationship of the cranial 

 nerves to the nine segments of the head. The olfactory and 



