THE MEDULLA OBLONGATA. 



upward, and limited by the striae medullaris. It is termed the trigonum 

 Jti/poglos-si, because it corresponds in position to the tract of nerve-cells from which 

 the hypoglossal nerve takes origin. A third triangular area to the outer side of 

 the fovea inferior, is named the trigonum acustici. It lies between the groove 

 forming the outer boundary of the fovea inferior and the lateral wall of the ventricle, 

 and, like the trigonum hypoglossi, has its base directed upward. Here it is con- 

 tinuous with a prominence, the tuberculum acusticum, which extends into the ante- 

 rior part of the floor of the ventricle. 



The superior triangle, or upper half of the floor of the fourth ventricle, i. e., the 

 part above the striae medullaris, presents in the middle line the continuation of 

 the median longitudinal sulcus. On either side of this is a spindle-shaped longi- 

 tudinal eminence, prominent in its centre, but less so above and below. This is 

 the eminentia teres, and is produced by an underlying bundle of white fibres, the 

 funiculus teres, formed, in part at all events, by the fibres of the facial nerve. 

 Immediately above and to the outer side of the eminentia teres is an angular de- 

 pression, the fovea superior ; this is sometimes crossed by a whitish band of fibres, 

 the conductor sonorus, which is connected beloAV with the striae medullaris. Above 

 the fovea is a bluish depressed area, the locus cceruleus. Its color is due to some 

 pigmented nerve-cells, showing through the white covering of the floor. These pig- 

 mented cells ai'e named the substantia ferruginosa, and in them one of the roots of 

 the fifth nerve terminates. 



The lining membrane of the fourth ventricle is continuous above with that of 

 the third, through the aqueduct of Sylvius, and below with that of the central 

 canal of the spinal cord. The cavity of the ventricle communicates below with the 

 subarachnoidean space by means of the foramen of Majendie and the foramina of 

 Key and Retzius, already described. 



Internal Structure of the Medulla Oblongata (Fig. 372). 



If the cranial nerves emerging from the medulla are traced into its substance, 

 it will be seen that they divide each half into three wedge-shaped areas, which are 

 named the anterior, lateral, and posterior areas of the medulla, and each of which 

 corresponds to one of the subdivisions already described on the surface of this por- 

 tion of the encephalon. 



The anterior area comprises that portion which is situated between the anterior 

 median fissure and the fibres of origin of the hypoglossal nerve. On the surface 

 of the medulla this area corresponds to the pyramid. 



The lateral area is situated between the fibres of origin of the hypoglossal nerve 

 on the one hand, and the fibres of the glosso-pharyngeal, pneumogastric, and spinal 

 accessory nerves on the other. On the surface of the medulla, in its lower part, 

 this area is single, and is called the lateral tract ; but in the upper part an oval- 

 shaped body, the olivary body, comes forward between it and the pyramid, pushing 

 aside the lateral tract. 



The posterior area comprises that portion which is situated between the fibres 

 of origin of the glosso-pharyngeal, pneumogastric, and spinal accessory nerves, and 

 the posterior median fissure. On the surface of the medulla this area is marked 

 by sl'ght furrows, splitting it up into smaller columns ; those in the lower part of 

 the medulla are named, from Avithout inward, the funiculus of Rolando, the funic- 

 ulus cuneatus, and the funiculus gracilis ; in the upper part of the medulla they 

 are replaced by the restiform body. Finally, the halves of the medulla are sepa- 

 rated from each other by a median septum or raphe'. 



Each of these three areas is made up of gray and white matter, the former 

 being derived for the most part from that of the cord. In like manner the white 

 matter is partly made up of longitudinal fibres continuous with those of the cord, 

 and partly of transverse fibres w r hich intersect them. 



In order to understand the internal structure of the medulla, it is necessary to 



