THE CENTRAL NERVOUS SYSTEM 



747 



themselves are unable to penetrate the marginal veil, the axis- 

 cylinder processes of some of them do so, and form the motor roots 

 of the spinal nerves. The neuroblasts from which the fibres of the 

 white columns of the cord are developed are apparently unable to 

 send their axons through the marginal veil. They are accordingly 

 forced to assume a longitudinal direction, and in this way the 

 central grey matter becomes covered with a sheath of longitudinal 

 white fibres. For a time only motor nerve-cells and the fibres 

 connected with them are developed in the cerebro-spinal axis. The 

 ganglia on the posterior roots arise from a series of ectodermic 

 thickenings or sprouts from the neural crest which runs along the 

 dorsal aspect of the neural canal. These sprouts contain the neuro- 

 blasts which develop into the spinal 

 ganglion cells with the posterior 

 root-fibres. From each pole of 

 each neuroblast a process grows out, 

 one towards the periphery, which 

 forms a peripheral nerve-fibre, the 

 other centrally to connect the cell 

 with the cord. From the after- 

 brain (or myelencephalon) is de- 

 veloped the medulla oblongata or 

 spinal bulb, from the hind-brain 

 (or metencephalon) the cerebellum 

 and pons, from the mid-brain (or 

 mesencephalon) the corpora quad- 

 rigemina and crura cerebri. The 

 fore -brain, or primary fore -brain 

 (thalamencephalon) gives rise of 

 itself only to the third ventricle 

 and optic thalamus, but a secondary 

 fore-brain (telencephalon) buds off 

 from it and soon divides into two 

 chambers, from the roof of which 

 the cerebral hemispheres, and from 

 the floor the corpora striata, are 

 derived. Their cavities persist as 

 the lateral ventricles, which com- 

 municate with the third ventricle by 

 the foramen of Monro. The olfac- 

 tory tracts are formed as buds from 

 the secondary fore-brain. 



To complete the story of the de- 

 velopment of the brain, it may be 

 added that the retina is really an 

 expansion of its nervous substance. 



A hollow process, the optic vesicle, buds out on each side from the 

 primary fore-brain. A button of ectoderm, which afterwards be- 

 comes the lens, grows against the vesicle and indents it so that it 

 becomes cup-shaped, the inner concave surface of the cup repre- 

 senting the retina proper, the outer convex surface the choroidal 

 epithelium. The stalk becomes the optic nerve. 



Histological Elements of the Central Nervous System. The central 

 nervous system is built up (i) of true nervous elements, (2) of 

 supporting tissue. The nervous elements have usually been described 

 as consisting of nerve-fibres and nerve-cells, but the antithesis of a 



FIG. 299. DIAGRAM TO ILLUSTRATE 

 THE FORMATION OF THE CEREBRAL 



VESICLES. 



A. i indicates the cavity of the 

 secondary fore-brain, which eventu- 

 ally becomes the lateral ventricles. 

 In B the secondary fore-brain has 

 grown backwards so as to overlap 

 the other vesicles. I, first cerebral 

 vesicle (primary fore-brain or 'tween 

 brain) ; II, second cerebral vesicle 

 (mid-brain) ; III, third cerebral vesi- 

 cle (hind-brain) ; IV, fourth cerebral 

 vesicle (after-brain). 



