THE SPINAL MARROW. 601 



of this layer pass up to the top of the brain, form a part of the 

 corpus callosum, and join with those of the opposite side^ The 

 middle triangular portion of the fornix, called the lyra, is now 

 developed between the extended portions of its crura, and the 

 divisions of the ventricles are completely formed. The middle 

 commissure of the thalami, nor the tenia striata are yet plainly 

 shown. The foetal human brain presents at this stage of its 

 growth, very much the same appearance, that it has in the 

 animals nearest to man ; with the exception that in the fcetal 

 brain the proportionate development of the cerebrum is much 

 greater. 



The cerebro-spinal axis may now be considered as completely 

 formed. The subsequent changes which occur during the eighth 

 and ninth months consist chiefly in its general growth, the 

 closing up of the cavity of the medulla spinalis with vascular 

 granular matter that after a time becomes ash-colored and forms 

 the nucleus of the medulla, and the development of the peri- 

 phery of the hemispheres, which having become too extensive 

 for the surface of the walls of the cranium, have their surfaces 

 folded inwards in the form of convolutions, so as to encroach 

 upon and nearly efface the cavity of the lateral ventricles. 



VOL. II. 51 



