XTIU'CTCHK Of Till: MEDULLA 797 



sion of I lie nucleus of termination of the vest ibular nerve (;tre:i aeustica). hut 

 Streelcr, who has made a detailed study of t he floor of the fourth ventricle by means 

 of serial sect ions, doubts that it is a part of this nucleus. 



Superior to the inferior fovea, and crossing each half of the floor of the fourth 

 ventricle, are the striae acusticae. These are bundles of a.xones arising in the nuclei 

 of termination of the cochlear division of the auditory nerve, which are situated in 

 the lateral periphery of the restiform body. The bundles course around the dorsal 

 periphery of the upper portion of the restiform body, giving it the appearance which 

 siitrgested its name, then across each half of the floor of the ventricle to the median 

 sulcus. in which they suddenly turn vent rally into the substance of the medulla oblon- 

 gata, and in doing so they cross the mid-line to enter the substance of the opposite side. 

 The striae acusticie vary greatly in different individuals, both in the degree of their 

 prominence and t heir direction. Frequently a bundle may be discerned which courses 

 obliquely upwards and outwards from the median sulcus to disappear in the floor fur- 

 ther away from the mid-line and again, a bundle may depart from the transverse 

 course before reaching the median sulcus. The stria 1 acust icsp cross the area acustica. 

 This is the flattened elevation which occupies the whole lateral portion of the interme- 

 diate portion of the floor of the ventricle, lateral to the limit ing sulcus, and extends into 

 the inferior portion lateral to the inferior fovea. It represents the subjacent nucleus 

 of termination of the vestibular division of the eighth cranial nerve. The dorsal 

 and ventral nuclei of the cochlear division of the eighth nerve (tuberculum acusti- 

 cum) 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 the 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 it pre- 

 sents a broader, well-marked, elongated elevation, the colliculus facialis. This repre- 

 sents 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 

 t his eminence is a depression of the limiting sulcus, which overlies 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 cseruleus. It also repre- 

 sents a portion of the nucleus of the trigeminus. 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 impor- 

 tant 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 modi 

 fied in varying degrees. 



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

 continuous into the pyramidal fasciculi of the spinal cord. They form the extreme 

 ventro medial portion of the medulla, and from the fact that they contribute numer- 

 ous 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 towards t lie spinal cord. Most of the fibres contributed to the medulk 

 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, con 

 tinues uncrossed into the spinal cord as the ventral cerebro-spinal fasciculus or direct 



