GU MIC CAigS 



and the roots of the vagus and glosso-pharyngcal nerves. The medial 

 part lies between the root fibres of the hypoglossal nerve and the median 

 raphe. It consists mainly of white matter, but on its ventral aspect, close 

 to the anterior median fissure, lies the nucleus of the external arcuate fibres, 

 embedded amongst those fibres ; and on the lateral part of the dorsal aspect 

 of the pyramidal fasciculus, there is a tract of nerve cells which is called the 

 medial accessory olive. 



The nerve fibres of the medial area are longitudinal, transverse and 

 oblique. The longitudinal fibres form four main strands. Ventrally they 

 form the pyramidal fasciculus ; immediately dorsal to the pyramidal fasci- 

 culus they constitute the medial lemniscus ; behind the lemniscus are the 

 tecto-spinal fibres, and still more dorsally, immediately subjacent to the 

 grey matter of the floor of the fourth ventricle, is the medial longitudinal 

 fasciculus. The medial lemniscus consists of the fibres of the spino-thalamic 

 fasciculi, prolonged upwards from the lateral and anterior funiculi of the 

 spinal medulla, and of internal arcuate fibres derived from the gracile and 

 cuneate nuclei of the opposite side. The tecto-spinal fibres are descending 

 from the lamina quadrigemina to the spinal medulla. The medial longi- 

 tudinal fasciculus is continuous, below, with the fasciculus anterior proprius 

 of the spinal medulla. It ascends through the medulla, pons, and mid-brain 

 to the subthalamic region, forming intimate associations with the motor 

 nuclei of the cerebral nerves. 



The oblique and transverse fibres of the medial area are internal and 

 external arcuate fibres. 



The most striking feature in transverse sections of the upper part of the 

 lateral area of the medulla oblongata is the olivary nucletts, which lies sub- 

 jacent to the olivary eminence. It presents the appearance of a thick 

 undulating layer of grey matter, folded on itself so as to enclose a space 

 filled with white matter and open towards the median plane. It is in 

 reality a grey lamina arranged in a purse-like manner with the open mouth 

 directed towards the raphe. Dorsal to the olivary nucleus lies the dorsal 

 accessory olivary mtcleus. More dorsally there are columns of nerve cells 

 which form the nucleus lateralis and the nucleus ambiguus, and which 

 are continuous, below, with the head of the anterior grey column of the 

 spinal medulla. Still more dorsally is the ventral part of the nucleus of 

 the spinal tract of the trigeminal nerve. 



The white matter of the lateral area consists of longitudinal and oblique 

 fibres, and that portion of it which lies dorsal to the olivary nucleus is 

 sometimes spoken of as \h&formatio reticularis grisea, because it contains a 

 certain amount of grey matter, whilst the corresponding part of the medial 

 area, which is practically devoid of grey matter, is called the form otto 

 reticularis alba. 



Some of the longitudinal white fibres of the lateral area of the medulla 

 oblongata form definite fasciculi which associate together different grey 

 masses. Close to the surface, below the level of the olive, and immediately 

 dorso-lateral to the issuing fila of the hypoglossal nerve lies the bulbo- 

 spinal fasciculus. On the superficial aspect of the olivary nucleus and 

 along its dorso-lateral border is the thalamo-olivary fasciculus, and dorsal 

 to the thalamo-olivary fasciculus are the ventral and the dorsal spino- 

 cerebellar fasciculi. More medially lie the rubro-spinal fasciculus and fibres 

 of the spino-thalamic fasciculi ; the oblique fibres are internal and external 

 arcuate fibres. Some of the internal arcuate fibres are passing between the 

 gracile and cuneate nuclei and the restiform body of the opposite side, and 

 some are connecting the cerebellar hemisphere of one side with the olivary 

 nucleus of the opposite side. 



The dorsal area of each half of the medulla oblongata also consists 



