STRUCTURE OF MEDULLA OBLONGATA 543 



the portion which corresponds to the basal part of the anterior column of the 

 spinal medulla is situated close to the median plane, whilst the part which 

 represents the base of the posterior column occupies a more lateral position. 

 Therefore the nucleus of origin of the hypoglossal nerve is placed in the 

 median part of the floor, whilst the nucleus of termination of the vagus 

 and glosso-pharyngeal nerves lies in the lateral part of the floor. 



The most conspicuous of the isolated clumps of grey matter in the 

 medulla are the olivary nucleus and the two accessory olivary nuclei. The 

 olivary nucleus lies subjacent to the olivary eminence, and is a very con- 

 spicuous object in transverse sections through this region. In such sections, 

 it presents the appearance of a thick wavy or undulating line of grey matter, 

 folded upon itself so as to enclose a space filled with white matter and open 



Floor of 

 ventricle iv. 



Ligula 



Vagus nerve 



- Medial 



longitudinal 



bundle 



Medial 



accessory. 



olivary 



nucleus 



Olivary 

 nucleus 



Pyramid 



FIG. 230. Transverse section through the Medulla Oblongata at the 

 level of the mid-point of the olive (Weigert-Pal stain). 



towards the median plane. It is in reality a lamina arranged in a purse- 

 like manner with its open mouth directed towards the raphe. 



The accessory olivary nuclei are two band-like laminoe of grey matter, 

 which are placed one on the dorsal and one on the medial aspects of the 

 main nucleus. 



Posterior to, or deeper than, the olive and pyramid is the formatto 

 reticularis of the medulla oblongata. It is divided into a lateral and a 

 medial field by the fila of the hypoglossal nerve as they traverse the sub- 

 stance of the medulla oblongata to reach the surface. In the lateral portion, 

 which lies posterior to the olive, there is a considerable quantity of grey 

 matter, continuous with that of the spinal medulla ; it is therefore called 

 the formatio grisea. In the medial part, however, which lies posterior 

 to the pyramid, the grey matter is extremely scanty, and the reticular 

 matter here is termed \\\Q formalio alba. 



