1064 



HTM AN ANATOMY. 



the anterior, lateral and posterior columns of the cord. This correspondence, how- 

 ever, is incomplete and only superficial, since, as will be evident after studying the 

 internal structure of the medulla, the components of the cord, both gray and white 

 matter, are rearranged or modified to such an extent that few occupy the same posi- 

 tion in the medulla as they do in the cord. 



The anterior median fissure is interrupted at the lower limit of the medulla, 

 for a distance of from 6-7 mm. , by from five to seven robust strands of nerve-fibres 

 that pass obliquely across the furrow, interlacing as they proceed from the two sides. 

 These strands constitute the decussation of the pyramids (decussatio pyramidum), 

 whereby the greater number of the fibres of the important motor paths pass to the 

 opposite sides to gain the lateral columns of the cord, in which they descend as the 

 lateral pyramidal tracts. The fibres that remain uncrossed occupy the lateral por- 

 tions of the pyramids and, converging towards the median fissure, descend on either 

 side of the latter within the anterior columns as the direct pyramidal tracts. The 



FIG. 915. 



Optic tract 

 Mammillary body 



Pons (basilar groove) 



Middle cerebellar peduncle 



Anterior median fissure 



Cerebellum 



Root-bundles of ninth 

 and tenth nerves 



Infundibulum 



Cerebral peduncle 



Interpeduncular space 



Tiigeminal nerve 



Middle cerebellar peduncle 



Inferior cerebellar peduncle 

 (Restiform body) 



Olivary eminence 



Arcuate fibres 

 Pyramidal decussation 



Root-bundles of twelfth nerve 



Anterior roots of first spinal nerve 



Brain-stem viewed from in front, showing ventral aspect < f medulla, pons and mid-brain. 



decussation varies in distinctness, sometimes the component strands being so buried 

 within the fissure that they are scarcely evident, or even not at all apparent, on the 

 surface and can be satisfactorily seen only when the lips of the groove are separated. 



Above the decussation the anterior median fissure increases in depth in conse- 

 quence of the greater projection of the bounding pyramidal tracts. Its upper end, 

 just below the inferior border of the pons, is marked by a slightly expanded triangular 

 depression, the foramen cffcunt. 



The posterior median fissure, the direct continuation of the corresponding 

 ur< H>ve on the cord, extends along only the lower half of the medulla, since above 

 that limit it disappears in consequence of (a) the separation and divergence of the 

 dorsal tracts of the bulb, which l>do\v enclose the fissure-, to form the lower lateral 

 boundaries of the lozenge-shaped fourth ventricle (fossa rhomboidalis), and (fi) 

 the gradual backward displacement of the central canal within the closed part of the 

 medulla until, at the lower angle of the ventricle, it opens out into that space. 



Kach half of the medulla is superficially subdivided into three longitudinal tracts 

 or areas by two grooves situated at some distance to the side of the ventral and dorsal 

 median fissures respectively. ( )ne of these, the antero-lateral furrow, marks the 

 line of emergence of tin- root-fibres of the hypoglossal nerve, which, being entirely 



