66o 



A MANUAL OF PHYSIOLOGY 



which the processes of the spongioblasts break up at the outside of 

 the primitive cerebro-spinal axis. Although the neuroblasts them- 

 selves 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 epiblastic thickenings arranged along the 

 neural canal, but outside its wall. From both poles of each ganglion 



cell 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 is developed the 

 medulla oblongata, from the 

 hind-brain the cerebellum and 

 pons, from the mid brain the 

 corpora quadrigeminaand crura 

 cerebri. The fore-brain, or 

 primary fore-brain, gives rise 

 of itself only to the third ven- 

 tricle and optic thalamus, but a 

 secondary fore-brain buds off 

 from it and soon divides into 

 two chambers, from the roof of 

 which the cerebral hemispheres, 

 OF THE CEREBRAL and from the floor the corpora 

 striata, are derived. Their 

 cavities persist as the lateral 

 ventricles, which communicate 



TO ILLUSTRATE 



A 3? 



FIG. 226. DIAGRAM 

 THE FORMATION 

 VESICLES. 



A. i indicates the cavity of the secondary 

 fore-brain, which eventually becomes the 

 lateral ventricles. In B the secondary fore- 

 brain has grown backwards so as to overlap with the third ventricle by the 

 the other vesicles. I, first cerebral vesicle foramen of Monro. The olfac- 

 (primiry fore-brain or tween brain); II, 

 second cerebral vesicle (mid-brain) ; III, 

 third cerebral vesicle (hind-brain) ; IV, fourth 

 cerebral vesicle (after-brain). 



tory 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 epiblast, 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 

 supporting tissue. The nervous elements have usually been described 



