STRUCTURE OF MEDULLA OBLONGATA 815 



nucleus of termination of the vestibular nerve. The dorsal and ventral nuclei of the cochlear 

 nerve [hibemdum acusticiim) are indicated by the ventro-lateral fullness in the contour of 

 the restiform body. In many of the mammals they produce a well-marked protuberance. 



In its superior portion the medial eminence occupies the greater part of the floor of the 

 fourth ventricle, and in the upper part of the intermediate portion of the floor it presents a 

 broader, well-marked, elongated elevation, the eminence of the facial and abducens or the 

 colliculus facialis. This represents the mesially placed nucleus of origin of the abducens and 

 the genu of the root of the facial nerve, which root courses around and above the nucleus of the 

 abducens. The nucleus of the facial is too deeply situated to produce an eminence. Lateral 

 to this eminence is a depression of the limiting sulcus, which overlies the mesial part of the region 

 of the larger portion of the nucleus of termination of the trigeminus, and is the fovea trigemini 

 or superior fovea. The strip of the floor above the superior fovea and lateral to the medial 

 eminence often appears greyish blue or dark brown, owing to pigmented cells subjacent to it, 

 and is known as the locus caeruleus. It also represents a portion of the nucleus of the trigem- 

 inus. The most superior portion of the medial eminence becomes narrow and lies close to 

 the mid-line. The function of the underlying grey substance producing it is uncertain, and 

 for this reason Streeter has named the elevation nucleus incertus, noting that by position it is 

 closely related to the upper portion of the nucleus of the trigeminus. 



Internal Structure of the Medulla Oblongata and Pons 



The finer detail of the internal structure lies within the scope of microscopic rather than 

 of gross anatomy. However, the significance and relations of certain of the more important 

 and larger of the internal structures of the medulla and pons as observed in sections may be 

 considered. 



The entire brain-stem may be regarded as an upward continuation of the spinal cord, to 

 which structures are added giving each part its peculiar character and conformation, and in 

 which the structures characteristic of the spinal cord are modified in varying degrees. 



The pyramids, the great descending or motor cerebro-spinal fasciculi, are directly con- 

 tinuous into the p5Tamidal fasciculi of the spinal cord. They form the extreme ventro-medial 

 portion of the medulla, and from the fact that they contribute numerous fibres to the efferent 

 nuclei (nuclei of origin) of the cranial nerves and to other portions of the grey substance of 

 the brain-stem, they decrease appreciably in bulk in descending toward the spinal cord. 

 Most of the fibres contributed to the medulla, as well as to other divisions of the brain-stem, 

 decussate as they leave the pyramids, and terminate in the grey substance of the opposite 

 side. However, the chief decussation of the pyramids occurs in the lower end of the medulla. 

 Here usually about three-fourths of the fibres then comprising the pyramids cross the mid- 

 line to form the lateral cerebro-spinal fasciculus (crossed pyramidal tract) of the spinal cord 

 immediately below. The remaining fourth, comprising the more lateral fibres or those furthest 

 away from the mid-line, continues uncrossed into the spinal cord as the ventral cerebro-spinal 

 fasciculus or direct pyramidal tract. The majority of the latter fibres decussate gradually 

 in the commissural bundle and in the ventral white commissure of the cord as they approach 

 the levels of their termination. In practically all vertebrates except man and the apes there 

 are no ventral pyramidal fasciculi, the decussation in the medulla being a total one. In man, 

 the proportion of fibres crossing in the chief decussation varies. Cases have been noted in 

 which apparently the entire pyramids decussate at this level. In other cases the direct or 

 ventral pyramidal tract may be much larger than usual, at the expense of the lateral. The 

 decussation usually appears to be symmetrical and it occurs so suddenly that the fibres, in 

 coursing from the ventral to the lateral positions, detach the tips of the ventral horns of the 

 spinal cord from the remainder of the grey figure, and these appear as isolated, irregularly 

 shaped masses of grey substance in transverse sections of the medulla. From this level upward 

 the outline of the grey figure of the cord is lost, and the cell-columns of the ventral horns occur 

 in more or less detached groups as the motor nuclei of the cranial nerves. 



The origin and decussation of the lemnisci (fiUet) begins immediately above the decussa- 

 tion, of the pyramids, and here the arrangements characteristic of the spinal cord are further 

 modified. The dorsal portion of the grey figure of the cord is manifest up to this level, but 

 here, after a considerable increase in its thickness, the grey commissure gives rise to two thick 

 dorsal outgrowths on each side of the mid-line. These dorsal projections of grey substance 

 comprise the nuclei of termination (relays) of the chief ascending or sensory spino-cerebral 

 fasciculi of the spinal cord. The nucleus of the fasciculus gracilis (nucleus of GoU's column) 

 arises a little before the nucleus of the fasciculus cuneatus (nucleus of Burdach's column). 

 The former extends slightly downward from its point of origin, so that its inferior extremity is 

 included in sections through the decussation of the pyramids (fig. 641). It produces a slight 

 bulbous enlargement (the clava) of the end of the funiculus gracilis, while the nucleus of the 

 fasciculus cuneatus corresponds to the cuneate tubercle of the external contour of the medulla 

 (figs. 632, 640). From the cells of these nuclei arise the lemniscus — the cephalic continuation 

 of the spino-cerebral pathway which conveys the general bodily sensations to the cerebrum. 

 In passing out of the nuclei the fibres of the lemniscus course in a ventro-medial direction. 

 Curving around the region of the central canal, they contribute largely to the internal arcuate 

 fibres, then, sweeping across the mid-line, they convert it into the raphe, and immediately after 

 crossing (decussating) they turn cephalad and collect to form the bundle known as the lemniscus. 



In the medulla, the lemnisci are two thin bands of fibres spread vertically on each side of 

 the raphe, with their lower or ventral edges thicker than their dorsal edges. In their course 

 toward the cerebrum they inc-ease in bulk, owing chiefly to fibres being added to them from 

 the nuclei of termination of the afferent roots of the cranial nerves, which fibres likewise cross 

 the mid-line as internal arcuate fibres to join the lemniscus of the opposite side. In passing 



