II92 



HUMAN ANATOMY. 



development of secondarj^ g>'ri and sulci, although the definite brain-pattern is not completed 

 until long after birth. 



Histogenesis of the Cerebral Cortex. — The changes in the walls of the brain-vesicles 

 incident to the development of the nervous elements of the cerebral cortex correspond 

 essentially with those occurring in the cord-segment of the neural tube (page 1049). The wall 

 of the pallium early differentiates into three zones : an inner layer, at first crowded with 

 closely packed and radially disposed proliferating cells ; an intermediate or mantle layer, 

 composed of more loosely and less regularly arranged cells ; and a narrow marginal layer, in 

 which nuclei are absent. The cells of the intermediate layer very soon are differentiated into 

 two kinds, which, in recognition of their fate, are known as the neuroblasts and the spongio- 

 blasts. Although both varieties are derived from the indifferent primary elements composing 

 the walls of the brain-tube, the spongioblasts are concerned in producing the sustentacular 

 tissue, the neuroglia, whilst the neuroblasts give rise to the neurones. The derivatives of the 

 spongioblasts become elongated into nucleated radial fibres, which by their numerous pro- 

 cesses form a supporting syncytium that at the inner and outer borders of the brain-wall 

 is condensed into the internal and the external limiting membrane respectively. The neuro- 

 blasts are soon distinguished by the outgrowth of a single and centrally directed process. 



Fig. 1030. 



Foramen of Monro, 

 Choroid plexus , 



Subthalamic region 

 Geniculate recess 

 Thalamus 



Pineal body 



Pallium 



Lamina terniinalis 



Rhinencephalon 



Tegmentum 



Mid-hrain 



Manjjnillary body 



Isthmus 



Cerebellum 



Corpus striatum 



Optic recess 

 Optic chiasm 



Pituitary body 

 Infunditnirar recess 



Tuber cinereuni 



Pons' 



Cervical flexure 



Spinal cord 

 Mesial surface of preceding reconstruction. Drawn from His model. 



which later is continued as the axis-cylinder of a nerve-fibre. They are further distinguished 

 by their peculiar affinity for stains, which deeply tinge the pointed ends of the cells from 

 which the axones are prolonged. A second process later grows from the young neurone in 

 the opposite direction, that is, towards the exterior of the brain, and becomes the peripherally 

 directed apical dendrite. The latter stains slightly and gradually invades the marginal layer. 

 After the appearance of the apical processes, the conversion of the neuroblasts into the 

 characteristic pyramidal cortical cells follows, so that by the end of the eighth week these 

 distinctive elements are recognized. The production of additional pyramidal cells is con- 

 tinued by the migration of neuroblasts from the nuclear layer. The subsequent formation 

 of the subcortical white matter follows the invasion of the inner part of the intermediate layer 

 by not only the axones of the pyramidal cells but by those of cells lying in more remote 

 parts of the brain, ingrowth of fibres taking place particularly from the thalamus. The young 

 nerve-fibres for a time are unprovided with medullary coats, the period at which' myelination 

 occurs marking the completion of the fibre as a path of conduction. The time at which 

 the fibres composing the various tracts within the brain acquire a medullary coat varies greatly. 

 In a general way, according to Flechsig, those constituting the corticipetal sensory paths first 

 myelinate ; then the projection-fibres from the sense-areas, and last of all the association strands, 

 which link together the sense-areas and the association fields. 



