Development of the Central Nervous System


Development of the Central Nervous SystemIn vertebrate embryos at a very early stage the central nervous system is laid down as a median neural groove on the dorsal surface. By the upgnowth and fusion of the lips of this groove a long neural tube results. Along either lip of the neural groove is a prominent thickening termed the neural crest; when the lips fuse a wedge-shaped ridge is formed along the line of closure.

As development proceeds, the neural tube is cut off from the general covering layer of the embryo by the growth round it of the skeleton of the vertebral column and skull.

The head end of the neural tube dilates to form three bulbs or vesicles, the fore-brain, mid-brain, and hind-brain vesicles respectively, which are ultimately modified to form the brain, while the rest of the tube becomes the spinal cord. The development of these portions need not be considered in detail; it will be sufficient to indicate the main modifications which they undergo.

Spinal Cord. - The nerve cells accumulate round the canal of the neural tube to form what is known, from its colour, as the grey matter of the cord. On the periphery of the grey matter, nerve fibres are grouped into tracts; these fibres at a later stage acquire white medullary sheaths and form the white matter of the cord. The original canal of the tube persists as the centra canal of the cord.

Hind-brain. - From the back part of the floor of the hindmost vesicle medulla oblongata or bulb, continuous with the spinal cord, is formed, and in front of this the pons Varolii develops. In the roof the cerebellum or little brain appears. The dilated cavity of the vesicle is termed the fourth ventricle.

Mid-brain. - In the floor of the middle vesicle the crura cerebri or peduncles of the brain are laid down. The roof of the vesicle develops four prominences called the corpora quadrigemina; in fishes, amphibians, reptiles, and birds there are but two of these prominences and they are known as the optic lobes. The cavity of the vesicle remains as a narrow channel, the iter, or aqueduct of Sylvius.

Fore-brain. - It is in the foremost vesicle that the most complex changes take place. The cavity of the posterior half of the vesicle dilates to form a vertical slit-like space - the third ventricle - which communicates behind with the fourth ventricle through the aqueduct of Sylvius. From the back part of the roof a small evagnatlon develops into the pineal body which in some of the lower animals, e.g. lizards, forms the rudimentary pineal eye.

In either lateral wall of the third ventricle larffe numbers of nerve cells accumulate to form a grey mass known as the thalamus. On either side a portion of the floor is evaginated in the form of a hollow bulb with a narrow stalk. The bulb is ultimately modifiod to form the retina - the portion of the eye which receives the stimulation of light; the stalk becomes solid and forms the optic tract and nerve. From the centre of the floor a downward evagination takes place: an evagination from the lining of the upper part of the primitive mouth grows up to meet it; this is cut off later from its site of origin, fuses with the median downgrowth from the third ventricle, and the two together form the pituitary body. Immediately behind this in the floor two little spherical bodies, lying side by side and known as the corpora albicantia or corpora mamillaria, are differentiated.

From the anterior half of the fore-brain vesicle four hollow outgrowths take place - two on either side. The first two are more or less cylindrical; they become completely solid as they grow and ultimately form the olfactory bulbs and tracts - structures concerned in the sense of smell. The other pair form the cerebral hemispheres which grow at a rapid rate and finally greatly exceed in size all the other parts of the brain. Into them the original cavity of the vesicle is continued on either side as a triradiate diverticulum. These two diverticula are termed the lateral ventricles (right and left); they communicate with each other and with the third ventricle through a Y-shaped foramen - the foramen of Monro.

The cerebral hemispheres are brought into communication with one another by transverse strands of fibres known as commissures ; the greatest of them is a broad transverse band called the corpus callosum.

In the wall of each lateral ventricle, forming a swelling projecting into its interior, is a large mass of grey matter termed the corpus striatum.

In the brain the grey matter is not, like that of the cord, entirely arranged, in a gular mass round the central canal. A certain amount of it does lie in the A the brain cavities, but separate portions exist at intervals amongst the fibres. In addition, the surfaces of the cerebral and cerebellar hemipheres are covered with grey matter.