THE BRAIN. 177 



By the growth of the brain at a much faster rate above 

 and below this depression its depth is increased, and as the 

 brain becomes larger the fissure becomes deeper and longer, 

 extending backward (the horizontal limb) with the backward 

 prolongation of the forebrain. Thus the fissure is formed 

 by the excessive growth of parts of the cerebrum about a 

 very slowly growing portion, so at the last the slowly grow- 

 ing portion is covered in by the more rapidly growing por- 

 tions, and there is left between them a fissure which leads 

 to the former part of the brain (in this case to the insula). 



As an example of the third class of depressions sulci 

 we will consider the formation of the fissure or sulcus of 

 Rolando. 



The first appearance of the fissure of Rolando is between 

 the fifth and sixth months, and it is produced by the growth 

 of the cortex of the cerebrum on either side of a linear 

 depression at a faster rate than the cortex grows along the 

 line of the depression itself. The result is to form a deep 

 fissure. All the rest of the third class of sulci are formed 

 in this manner by the upgrowth of the cortex at a faster 

 rate along certain lines than along other lines, these up- 

 growths being called convolutions and the depressions sulci. 



The Formation of the Velum Interpositum and the Choroid 

 Plexuses. (Diags. 13 to 16.) Go back to the time when 

 there are the three primary vesicles ; cover them with the 

 vascular membrane, which is to become the adult pia. Now 

 remember how the forebrain grows backward, taking with 

 it the lamina terminalis (corpus callosum), so as to cover in 

 the middle vesicle. The forebrain does cover in the middle 

 vesicle or that portion of it which we call its roof, but upon 

 this roof rests the vascular membrane (which later we 

 designate as the velum interpositum), so at the end of the 

 process the velum interpositum is found within the brain, 



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