THE BRAIN. 173 



axis of these masses lies anteroposteriorly. The most 

 anterior (and external) mass of cells becomes the adult 

 corpora striata, the posterior (and internal) mass, the optic 

 thalami. These masses are at first separated from each 

 other, but as growth goes on they become united along 

 their opposing surfaces. Furthermore, the primitive corpus 

 striatum is separated from the lateral wall of the brain 

 vesicle, but as development proceeds the two approach and 

 finally fuse, but this line of junction is always a weak spot 

 in the brain. Along this line of junction the lenticulo- 

 striate artery passes, and that this line of junction is a weak 

 spot is proven by the fact that cerebral hemorrhage takes 

 place here from this artery more often than at any other 

 point. The effused blood separates the apparently united 

 corpus striatum from the vesicular wall. 



The fore part of the interbrain which unites the fore- 

 brains of opposite sides is called the lamina terminalis. By 

 the extension of this backward along with the growth of 

 the forebrain and by the development in it of fibres which 

 connect the forebrains, the great median commissure 

 corpus callosum is formed. 



The portion of the lamina beneath the corpus callosum 

 is projected backward in the shape of cylindrical bundles 

 of fibres which are called the fornix. 



Between the fornix and the corpus callosum the lamina 

 terminalis persists as a triangular shaped, very thin layer 

 of brain matter, which helps to separate the two lateral ven- 

 tricles and is called the septum lucidum (pellucidum). 



Within the septum lucidum by a process of vacuolation 

 a cavity is formed which in the adult is called the fifth ven- 

 tricle. It has no connection with the other ventricular 

 cavities of the brain which are developed from the original 

 cerebrospinal canal. 



