THE MEDULLA OBLONGATA 253 



The fourth ventricle deserves particular attention. It is 

 a cavity on the posterior aspect of the pons and medulla ex- 

 tending from the upper limit of the former to a point on the 

 latter opposite the lower border of the olivary body. It has 

 the shape of two isosceles triangles placed base to base. The 

 apex of the inferior triangle is at the calamus scriptorious, 

 and its lateral boundaries are the diverging restiform bodies. 

 The superior peduncles of the cerebellum form the lateral 

 boundaries of the superior triangle. The inferior triangle 

 is covered by the cerebellum; the superior by the valve of 

 Vieussens, which stretches between the superior peduncles. 

 This ventricle communicates above with the third ventricle 

 by the aqueduct of Sylvius, or the iter a tertio ad quartum 

 ventriculum; below, with the central canal of the cord and 

 with the subarachnoid space. The floor of the ventricle 

 presents a longitudinal median fissure and numerous small 

 elevations indicating the position of the nuclei of origin of 

 certain of the cranial nerves. 



The gray matter of the medulla has the same general dis- 

 tribution as that in the cord, but is by no means so regular in 

 its disposition. The direction of the white fibers is not so 

 uniform as in the cord. They run not only longitudinally, but 

 transversely to connect the lateral halves, and in other direc- 

 tions to connect various centers situated in this part of the 

 encephalon and to connect the medulla with other parts of 

 the brain. The following is the relation of the columns of 

 the cord to the medulla : 



The direct and crossed pyramidal tracts pass to the ence- 

 phalon constituting, in the medulla, the anterior pyramids 

 the direct, having decussated below, occupying here the outer 

 portion of the pyramid, and the crossed decussating in the 

 medulla and occupying the inner portion of the pyramid. 



Those columns concerned in reflex action, the anterior 

 fundamental fasciculi, the anterior root zones and the mixed 

 lateral tracts do not continue farther upward than the gray 

 matter of the medulla. 



