ITERNAL STRUCTURE OF MEDULLA OBLONGATA AND PONS. 551 



n nner the portion of the floor which lies below the striae medullares is mapped out 

 i:o three triangular areas. The medial subdivision is slightly elevated and is 

 t cued the trigonum nervi hypoglossi, because subjacent to the medial part of this 

 a a is the nucleus of origin of the hypoglossal nerve. The intermediate area, 

 1; ween the two diverging grooves which proceed from the base of the fovea 

 i erior, is the ala cinerea. It is sometimes called the trigonum n. vagi because 

 tj nucleus of the vagus and the glossopharyngeal nerves lies subjacent to it. 

 Lar the lateral angle is the area acustica. The base of this area is directed 

 i wards and runs directly into an eminence over which the strise medullares 

 ps. Subjacent to this district of the floor of the ventricle lies the large terminal 

 c.ef nucleus of the vestibular division of the acoustic nerve. A more accurate 

 line for the area acustica would be area vestibularis. 



A close inspection of the pars inferior fossae rhomboidese will show that the base of the 

 t ,'onum vagi is separated from the medial margin of the clava by a narrow lanceolate strip of 

 t ventricular floor, to which Ketzius has given the name of area postrema. Beneath this area 

 i ome vascular tissue (Streeter), and marking it off on its superior and medial aspect from the 

 1 e of the trigonum vagi there is a translucent cord-like ridge called the funiculus separans. 



When the floor of the ventricle is examined under water with a magnifying glass, the 

 t Tontun hypoglossi is seen to consist of a narrow medial strip which corresponds to the hypo- 

 ssal nucleus, and a wider lateral part which has been shown to be the surface representation 

 c mother nucleus termed the nucleus intercalatus (Streeter). 



On the part of the floor of the ventricle which lies above the striae medul- 



]-es, and corresponds to the dorsal surface of the pons, there is also a slight 



<pression, termed the fovea superior. Between it and the median groove is a 



: irked prominence called the eminentia medialis. Inferiorly this elevation passes 



i wnwards and becomes continuous with the trigonum hypoglossi, whilst superiorly 



is carried upwards towards the opening of the aquseductus cerebri. In both 



sections it gradually becomes less prominent, but still it forms a distinct 



mgated elevation, which stretches alongside the whole length of the median 



oo ve. As already stated, the area acustica extends upwards into the pontine 



rt of the ventricular floor and forms an elevated region in the most lateral part 



its widest portion, below and to the lateral side of the fovea superior. Proceed- 



g upwards from the fovea superior to the opening of the aquseductus cerebri 



ere is a shallow depression termed the locus coeruleus, seeing that it usually 



esents a faint slate-blue colour. When the ependyma is scraped away from 



e surface of this part of the floor, the colour is seen to be due to the substantia 



rruginea, a name applied to a linear group of strongly pigmented cells, which 



)s in the lateral part of the gray matter covering this portion of the ventricular 



>or. When transverse sections are made through the superior part of the pons, 



e substantia ferruginea appears on the cut surface as a small black spot or dot. 



XTERNAL STRUCTURE OF MEDULLA OBLONGATA AND PONS. 



The structure of the medulla oblongata and pons differs in a marked degree from 



iat of the spinal medulla : indeed, in its superior part, it presents very little in 



unmon with the latter. Some of the largest fasciculi which come up from the spinal 



edulla (such as the funiculus posterior) end in the lower part of the medulla 



)longata ; others leave the medulla oblongata and pass into the cerebellum ; and 



the bundles of fibres which pass upwards or downwards, from or to the spinal 



edulla respectively, most of them come to occupy very different positions in the 



-edulla oblongata and pons. 



The gray matter instead of being moulded into one compact column, as is the 



1 in the spinal medulla, becomes broken up into a series of discrete nuclei. 



there are developed from the basal lamina of the rhombencephalon not one 



nipact mass like the spinal anterior column, but three distinct broken columns 



fferent nuclei (Fig. 526, p. 593) : (1) a medial somatic column, which in turn is 



ken up into two parts, a bulbar nucleus (the hypoglossal) which supplies the 



fibres to the tongue muscles, and a pontine nucleus (the abducens) which 



the lateral rectus muscle of the eye ; (2) a lateral somatic column, broken 



p into separate nuclei, viz., accessorius, ambiguus, facial, and trigeminal, supplying 



36 c 



