822 THE CENTRAL NERVOUS SYSTEM 



ginal veil, a closely- woven network or thicket, into which the processes of 

 the spongioblasts break up at the outside of the primitive cerebro -spinal 

 axis. Although the neuroblasts 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 neuroblasts 

 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 developed the medulla oblongata or spinal bulb, 

 from the hind-brain (or metencephalon) the cerebellum and pons, 

 from the mid-brain (or mesencephalon) the corpora quadrigemina and 

 crura cerebri. The fore-brain, or primary fore-brain (thalamencepha- 

 lon), 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 hemi- 

 spheres, and from the floor the corpora striata, are derived. Their 

 cavities persist as the lateral ventricles, which communicate with the 

 third ventricle by the foramen of Monro. The olfactory tracts are 

 formed as buds from the secondary fore-brain. 



To complete the story of the development 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 becomes 

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

 cup-shaped, the inner concave surface of the cup representing 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 sup- 

 porting tissue. The nervous elements have usually been described as 

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

 honoured distinction must not lead us to forget that the essential 

 part of a nerve-fibre, the axis-cylinder, is a process of a nerve-cell, 

 and the medullary sheath a structure whose integrity is intimately 

 related to that of the axis-cylinder.* In strictness, the term ' nerve- 

 cell ' ought to include not only the cell-body, but all its processes, out 

 to their last ramifications. But the habit of speaking of the position 

 of the cell-body f as that of the nerve-cell is so ingrained, that it seems 

 better to continue the use of the latter term in its old signification, and 

 to speak of the cell and branches together as a neuron (also spelled 

 neurone). 



* While the medullary sheath, like the axis-cylinder, seems to be as regards 

 its nutrition under the control of the nerve-cell, and must therefore be looked 

 upon as an integral portion of the neuron, although not essential for its de- 

 velopment, the neurilemma in respect both of its nutrition and its develop- 

 ment appears to be an independent structure. 



t Foster and Sherrington call the cell-body the perikaryon. 



