GRAY MATTER OF MEDULLA. 303 



93). This is the nudes arcuatus. It forms a large crescentic 

 mass on the ventral and medial surface of the pyramid. Run- 

 ning over and through it there are the anterior external arcuate 

 fibers, for certain of which it constitutes a relay. The arcuate 

 nucleus is continuous above with the nucleus ponds. It is brought 

 to the surface at the lower border of the pons by the disappear- 

 ance of most of the transverse fibers. 



(2) The gray substance in the substantia reticularis and in 

 the immediate floor of the fourth ventricle (Figs. 92 and 93) 

 is continuous with the same in the pons and mid-brain, above; 

 and is represented in the spinal cord by the H-shaped column 

 of gray matter. By the posterior and lateral expansion of the 

 neural canal, in the upper half of the medulla and in the pons, 

 which forms the fourth ventricle, the posterior columnae of the 

 H-shaped column of gray matter are pushed outward to a trans- 

 verse direction; and the entire bases of the anterior columnas are 

 brought into the floor of the ventricle. The expansion of the canal, 

 together with the decussation of the lateral pyramidal tracts through 

 the anterior columnae and of the medial fillets through the pos- 

 terior and anterior columnae, disposes the H-shaped column as 

 follows : 



Anterior Columna. From the base of the anterior columna is 

 derived a column of cells, the hypo glossal nucleus (Figs. 92 and 93), 

 which is two-thirds of an inch in length and extends along the 

 median raphe in the upper medulla, beneath the eminentia media- 

 lis. It is continued into the lower medulla as far as the pyram- 

 idal decussation. By commissural fibers it is joined to the nu- 

 cleus of the opposite side according to Kolliker. The nucleus 

 gives origin to the hypoglossal nerve proper; and probably to a 

 small fasciculus which by way of the medial longitudinal bundle 

 joins the facial nerve and supplies the orbicularis oris. The 

 hypoglossal axones run in linear series forward through the me- 

 dulla to the anterior lateral sulcus, whence they emerge between 

 the pyramid and the olive (Fig. 85). They separate the anterior 

 from the lateral area. The main body of the anterior columna 

 is broken up into the nucleus later alls inferior, the nucleus ambi-g- 

 uus and the motor part of the nucleus of the ala cinerea. 



