BRAIN 



105 



columns corresponding with the tracts of conduction ; the last of these tracts to 

 become medullated are the pyramidal tracts. 



FIG. 142. SECTIONS ACBOSS THE BEGIOX OF THE CALAMUS SCRIPTORIUS OF THE BRAIN REPRESENTED 



IN FIG. 141, A. (His.) 

 A, region of the glosso-pharyngeal ganglion. B, region of the auditory facial ganglion. 



The membranes are formed from the mesenchyme of the sclerotomes, which 

 extends over and under the cord and becomes enclosed along with that structure 

 within the developing vertebral canal. The septa of connective tissue which are 



bl 



FIG. 148. SECTIONS ACROSS THE FOURTH VENTRICLE OF A SOMEWHAT OLDER EMBRYO. (His.) 



A, section taken through the lower part. B, across the widest part (trigeminus region). 



C, through upper part (cerebeilar region). 

 r, roof of neural canal ; al, alar Jamina ; bl, basal lamina; v, ventral border. 



seen penetrating into the substance of the cord from the pia mater grow in from 

 this mesenchyme, carrying blood-vessels amongst the nervous elements. 



FIG. 144. SECTIONS ACROSS THE LOWER HALF OF THE RHOMBENCEPHALON OF A STILL OLDER EMBRYO, 

 SHOWING GRADUAL OPENING OUT OF THE NEURAL CANAL AND THE COMMENCING FOLDING OVER OF 



THE ALAR LAMINA (at /). 



v, ventral border; t, tsenia : ot, otic vesicle; r.L, recessus labyrinthi. 



In the succeeding stage (not here represented) the angle at v has almost disappeared, the fold / 

 {rhombic Up) has extended over the alar lamina, and the two thickened halves are in the same 

 horizontal plane, covered by a greatly expanded and thinned-out roof. 



BRAIN, The cephalic part of the neural tube closes rather later than the spinal 

 part. In some mammals (pig ; Keibel) the medullary laminae close only after 

 organogenesis has proceeded to some extent, and after the cephalic flexure has 



