528 THE HUMAN EMBRYO. 



run back some distance along the vertebral canal before reach- 

 ing their foramina of exit. 



3. The Cranial Nerves. 



The structure of the brain in the early stages of development, 

 and the sequence of changes which it undergoes, are similar to 

 those of the spinal cord in all essential respects. 



In the latter part of the third week a myelospongium, or 

 epithelial framework is formed : in this an outer or mantle layer, 

 containing iieuroblasts, can early be distinguished from a thicker 

 inner plate in which lie the nuclei of the spoiigioblasts. The 

 neuroblasts give rise to axis-cylinder processes, which either 

 collect in bundles and grow out from the brain as the motor 

 roots of the cranial nerves, or else run in the substance of the 

 brain, longitudinally, obliquely, or transversely, to form the tracts 

 of white matter, or nerve fibres, which connect the brain with 

 the cord, and the several parts of the brain with one another. 

 Other bundles of nerve fibres enter the brain by growing into 

 it from the ganglia of the sensory cranial nerves. 



Histological differentiation is established in the medulla 

 oblongata even earlier than in the spinal cord. In the cerebral 

 hemispheres it does not appear until a comparatively late stage 

 of development. All the cranial nerves are definitely formed by 

 the end of the fourth week (Fig. 227). 



The cranial nerves are more difficult to deal with than the 

 spinal nerves, on account of their want of uniformity in arrange- 

 ment, and the great differences in size and in relations which 

 they present among themselves. 



With the possible exception of the optic nerve, however, it 

 appears that the cranial, like the spinal nerves, may be divided 

 into two categories : 



(i) Centrifugal or motor nerves, which are formed by out- 

 growth of axis-cylinder processes from groups of neuroblasts 

 situated in the brain itself. 



(ii) Centripetal or sensory nerves, which are formed by out- 

 growth of axis-cylinder processes from groups of neuroblasts 

 situated, not in the brain, but in the sensory ganglia outside the 

 brain ; the processes growing in two directions, inwards into the 

 substance of the brain, and outwards to the peripheral distribu- 

 tion of the nerve. 



