798 MAN 



and nucleus of Burdach). A dorsolateral sulcus separates the cuneus from a 

 well-defined eminentia trigemini. These three elevations of the dorsal sur- 

 face constitute the major elements entering into the dorsal sensory field. In 

 relatrve size the column of Burdach has increased in its diameters as com- 

 pared with the column of Goll. This expansion indicates an increment in the 

 influx of sensorj- impressions from the upper extremity and hand as compared 

 with those making their way brainward from the lower extremity and foot. 

 At their cephalic extremity the clava and cuneus become reduced in their sur- 

 face relief. The cuneus extends further cephalad than the clava and at the 

 level of the lateral recess of the ventricle reaches the general plane of the 

 ventricular floor. The superior angle of the fourth ventricle is bounded upon 

 either side by the middle and superior cerebellar peduncles and decreases in 

 size as it approaches the caudal orifice of the Sylvian aqueduct. The floor 

 of the fourth ventricle thus outlined consists of two triangles, the inferior 

 triangle bounded by the cuneus and clava, and the superior triangle bounded 

 by the middle and superior cerebellar peduncles. 



At the caudal angle of the fourth ventricle the divarication of the alar 

 plates is indicated by a remnant of the gray matter, the obex. The floor is 

 divided into two symmetrical halves by a median sulcus w hich runs from the 

 apex of the lower triangle to the apex of the upper triangle. A second sulcus 

 starts beneath the obex lateral to the median sulcus, and proceeds cephalad. 

 It is situated about midway between the median sulcus and the lateral wall. 

 This is the sulcus limilans which divides the floor of the ventricles into a 

 mesial motor portion derived from the basal plate, and a lateral sensory 

 portion derived from the alar plate. At the caudal angle of the ventricle 

 immediately adjacent to the midline is a long narrow elevation, the eminentia 

 hypoglossi, which indicates the cephalic extremit}- of the hypoglossal nucleus. 

 Cephalad to the eminentia hypoglossi and upon either side of the median 

 sulcus is a less prominent elevation, the eminentia fasciculi teretis. In a posi- 



