THE BRAIN AND ITS MEMBRANES. 709 



is clearly marked off from the pons by prominent transversely directed fibres 

 belonging to the latter ; dorsally, however, there is no such line of separation, 

 the dorsal surface passing directly and smoothly into that of the pons. The 

 length of the medulla is nearly 1 inch (20 to 24 mm.) ; its greatest lateral 

 diameter is about three-quarters of an inch (17 to 18 mm.) ; its greatest dorso- 

 ventral diameter is somewhat less (15 mm.). % 



The further description of the medulla will be divided into that of its surface 

 and that of its internal structure. 



Surface. 



The Surfaces of the Medulla. The ventral surface of the medulla is divided 

 into two symmetrical lateral halves by the continuation upward of the anterior 

 median fissure of the spinal cord. This continues up to the pons, where it ter- 

 minates in a recess, the foramen cacum of Vicq d'Azyr. It is interrupted, how- 

 ever, for a short distance after its passage into the medulla by the decussation of 

 the crossed pyramidal tracts of the cord. The dorsal surface of the lower half 

 of the medulla is similarly divided by the posterior median fissure of the cord, 

 which does not extend on to the dorsal surface of the upper half. This surface 

 is, however, bisected by a groove or sulcus which lies in the middle line and 

 extends from the junction of the upper and lower halves of the medulla on to 

 the dorsal surface of the pons as far as its upper extremity. 



Each lateral surface of the medulla is separated from the adjacent halves of 

 the dorsal and ventral surfaces by a groove, well marked above, less distinct 

 below. These grooves may be called, respectively, the dorso-lateral and ventro- 

 lafcral grooves of the medulla. 



The dorso-lateral groove is the continuation upward of the postero-lateral 

 groove of the cord, and from it emerge the fibres of the accessory portion of the 

 spinal accessory nerve, of the vagus, of the glosso-pharyngeal, and, from the 

 extreme upper part, close to the pons, the fibres of the seventh and mesial root 

 of the eighth. There are two points to be noted in connection with this groove : 

 First, it is interrupted at its lower end by the change in position of the direct 

 cerebellar tract of the cord. In the cord this tract is anterior to the postero- 

 lateral groove, but as it passes upward into the medulla it becomes dorsal to the 

 groove, and thus belongs to the corresponding half of the medullary dorsal sur- 

 face. Secondly, its direction is not straight up and down, but is upward, for- 

 ward, and outward. The reason for this change of direction, as well as for the 

 cessation of the posterior median fissure, will be explained below. 



The ventro-lateral groove is the direct continuation upward of the line of 

 emergence of the anterior roots of the spinal nerves, although in the cord there 

 is no similar sulcus. Out of this groove, in the upper half of the medulla, whre 

 it is very distinct, pass the fibres of the hypoglossal nerve. 



These various surfaces will now be considered in detail. 



THE VENTRAL SURFACE OF THE MEDULLA. Its lower half is made up, 

 mesially, of the decussation of the crossed pyramidal tracts, and, laterally, of 

 the continuations upward of the direct pyramidal tracts of the cord. Hence it 

 is undivided, and extends laterally from the lower part of one ventro-lateral 

 groove of the medulla to the other. Its upper half is divided in two, as already 

 stated, by the anterior median fissure. These two halves are known as the 

 pyramids. 



The pyramids are two prominent, somewhat pyramidally shaped bundles of 

 white matter or nerve-fibres, placed one on either side of the anterior median 

 fissure, each being separated from the upper half of the corresponding lateral 

 surface by the upper part of the ventro-lateral groove. Superiorly, they reach 

 to the pons, at the lower border of which they are somewhat constricted. Each, 

 as it descends, becomes somewhat enlarged, and then tapers at its lower extrem- 

 ity. The fibres of which each pyramid is composed are disposed in two bundles, 



