68 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



first month of post-foetal life a few fibres of the anterior central 

 convolution receive their medullary sheaths. Later, in the second 

 to third month, the medullary sheaths are developed in the 

 cortex of the occipital lobe. It is probable that these conditions 

 correspond to the time when the individual first begins to retain 

 memory-pictures in the different regions of the brain ; that, for 

 instance, the cortex of the visual centres is developed as soon as 

 conscious visual impressions are received. 



In later life still more extensive tracts become medullary. 

 After the third year it is impossible to trace this increase for the 

 reason that the medullated fibres have already become too univer- 

 sal. Nevertheless, there is a great probability that this additional 

 development of medullated fibres continues for many years, and 

 that new nerve-tracts are continually developed as new regions 

 of the cortex are brought into activity. 



Below the cortex lies the white substance of the hemi- 

 spheres. The homogeneous white mass which is revealed by a 

 section through the centrum semiovale is resolved by the micro- 

 scope into a great number of fibres crossing each other in every 

 direction, and defying our attempts to differentiate them. Let 

 us endeavor to ascertain what is at present known about these 

 white fibres. 



If we make a section through the fresh brain of a newborn 

 child we shall see that below the cortex there lies almost every- 

 where a peculiar, glistening, reddish-gray mass, in which at only 

 one point underneath the posterior central convolution and its 

 vicinity medullary fibres can be found. It is only in the course 

 of the first months of the post-natal period that other nerve- 

 tracts become invested with medullary substance. This takes 

 place first in those bundles of fibres which pass downward from 

 the cortex, afterward in those which connect the different regions 

 of the cortex with each other. The latter, the fibrre proprise 

 of the cortex, are especially abundant in the adult brain. They 

 extend everywhere, from convolution to convolution, connecting 

 parts which lie near each other as well as those which are widely 



