STRUCTURE OF MEDULLA OBLONGATA. 737 



The deep arciform fibres (h) are more numerous, and are seen over the and deep, 

 whole area of transverse sections except in the pyramid. Some of them come 

 to the surface on the inner side of, and through the olivary body, and join 

 the superficial set. Others are deep in their whole extent, and pass outwards 

 into the restiform body, and to the nuclei of the cuneate and slender funiculi. 

 Internally, the arciform fibres enter the rapbe, through which they are 

 continued to the opposite half of the bulb. 



The raphe (between h and /) occupies the median plane of the medulla Raphe. 

 oblongata above the decussation of the pyramids, and consists of fibres running 

 obliquely, longitudinally, and from before backwards, which are in part con- 

 tinuous with the superficial and deep arciform fibres, and in part spring from 

 the nuclei in the floor of the fourth ventricle. 



Fvi'iiKitio reticularis. In the dorsal portion of the medulla oblongata, Reticular 

 behind the pyramid and olivary body (I and &), the longitudinal fibres formation, 

 derived from the anterior and lateral columns of the cord, decussating with 

 the deep arciform fibres, give rise to a structure that is known as the 

 recticular formation of the bulb. In the part of the reticular formation 

 behind the olivary body (k) interspersed grey matter containing nerve-cells 

 is also present. 



Olivary body (fig. 265, /). On removing a thin slice from the Olivary 

 olivary body, it will be seen to consist of three parts, viz., an bn 

 external investment of white substance, a thin grey layer, the 

 olivary nucleus, and a central white core. 



The outer white layer consists mainly of transverse fibres, which 

 belong to the superficial arciform group. 



The olivary nucleus or corpus dentatum is a thin plaited capsule its nucleus, 

 or bag, having a zigzag outline in section. Towards the surface 

 and behind it is closed, but on the inner side it is open, forming 

 a narrow neck, which is turned towards the raph.6, and gives 

 passage to the olivary peduncle. 



The central white matter fills the grey capsule, and is formed 

 by the spreading out of a tract of white fibres called the olivary and 

 l>tdiuide, which passes inwards through the opening in the nucleus pe 

 to the raphe". The fibres of the peduncle partly terminate in the 

 cells of the corpus dentatum, and are partly continued through the 

 grey layer to join the arciform fibres. 



Grey matter of the medulla oblongata. The larger part of the grey matter Grey matter 

 in the bulb is a continuation of that of the cord, but there are in addition of bulb 

 some smaller independent masses. 



Prolongation of yrey matter of the spinal cord. At the lower end of the prolonged 

 medulla oblongata the central grey matter resembles that in the spinal cord, fr m cord, 

 but as it extends upwards it undergoes the following changes : 



The anterior cornu is broken up by the passage through it of the fibres of Changes in 

 the crossed pyramidal tract, and the detached extremity of the horn is anterior 

 continued upwards in the lateral tract for some distance as the lateral nucleus. horn ' 



The posterior cornu is pushed outwards by the increasing development of in posterior 

 the gracile and cuneate funiculi, and its extremity (caput), consisting of the horn, 

 substantia gelatinosa of Rolando, becomes greatly enlarged aud approaches 

 the surface, giving rise to the funiculus and tubercle of Rolando (fig. 265, c). 

 From the base of the horn, processes of the grey matter extend backwards 

 into the slender and cuneate funiculi, and are known as the nuclei of those 

 bodies. They are largest in the neighbourhood of the lower end of the fourth 

 ventricle, where they cause the swellings which have been described above 

 as the clava and cuneate tubercle (p. 733). 



By the opening out of the posterior median fissure and central canal of the and in 

 cord to form the fourth ventricle, the anterior portion of the grey commissure commissure. 



D.A. 3 B 



