PARTS DERIVED FROM THE MID-BRAIN 313 



above and external to the latter and the eminentia abducentis 

 is the fovea and area trigemini; the area vestibularis is the 

 smooth surfaces above and below the striae acusticoe and laterad 

 of the lateral furrow, connecting the .superior and inferior 

 fovea. The pars inferior presents in the centre a longitudinal 

 furrow dividing it into two small triangles, with their bases 

 placed cephalad. They present just adjacent to the furrow 

 and below the striae acusticse the eminentia hypoglossi and 

 the nucleus intercalates ; external to and below the latter is a 

 depression, the fovea inferior, and below the fovea inferior 

 is an eminence, the trigonum vagi, or ala cinerea, which is sepa- 

 rated from the area postrema, by the funiculus separans. 

 The convergence of the median and lateral furrows at the 

 caudal apex of the rhomboidal fossa forms the calamus 

 scriptorius, which resembles an ancient writing reed or quill 

 pen (Gray); hence the name. Caudad the fourth ventricle is 

 continuous with the small central canal of the cord and post- 

 oblongata (in part); cephalad it communicates with the third 

 ventricle by means of the aqueduct, or mesocele. The fourth 

 ventricle has an opening through the tela choroidea, which 

 permits of communication with the subarachnoid space, and it 

 is called the foramen of Majendie; also, the extremities of the 

 lateral recesses permit of a tidal flow of the cerebrospinal 

 fluid, through the foramina of Luschka. The locus caeruleus 

 is continued upward from the superior fovea and extends 

 well up into the aqueduct; it owes its color to the refraction 

 of the pigmented cells, the substantia ferruginea. 



Parts Derived from the Mid-brain (Mesencephalon) 



The mid-brain and the parts included therein the crura 

 cerebri, corpora quadrigemina, the internal geniculate bodies, and 

 the aqueduct (mesocele) and central aqueduct gray. 



The crura cerebri are seen, after separating the temporal 

 lobes, as two white bundles, which emerge above the pons; 

 diverging, they pass forward and outward to the inner and 

 under part of each hemisphere; beneath the optic tract. Be- 

 tween them is seen the intercrural (interpeduncular or posterior 

 perforated space) ; near the inner border of each crura are the 

 roots of the motor oculi nerves passing forward from the sulcus 

 oculomotorius. 



