512 



NERYOTJS SYSTEM. 



Fig. 261. Posterior Surface of Medulla 

 Oblongata. 



bodies by a narrow groove. They consist entirely of white fibres, and are con 

 tinuous with the posterior median columns of the spinal cord. These bodies lie 



at first in close contact. Opposite the 

 apex of the fourth ventricle, they form 

 an enlargement, the processus clavatus, and 

 then, diverging, are lost in the correspond- 

 ing restiform body. The upper part of 

 the posterior pyramids forms the lateral 

 boundaries of the calamus scriptorius. 



The posterior surface of the medulla 

 ollongata forms part of the floor of the 

 fourth ventricle. It is of a triangular 

 form, bounded on each side by the di- 

 verging posterior pyramids, and is that 

 part of the ventricle which, from its 

 resemblance to the point of a pen, is 

 called the calamus scriptorius. The di- 

 vergence of the posterior pyramids and 

 the restiform bodies opens to view the 

 gray matter of the medulla, which is con- 

 tinuous, below, with the gray commissure 

 of the cord. In the middle line is seen a 

 longitudinal furrow, continuous with the 

 posterior median fissure of the cord, ter- 

 minating, below, at the point of the ven- 

 tricle, in a cul-de-sac, the ventricle of 

 Arantius, which descends into the medulla 



for a slight extent. It is the remains of a canal, which, in the foetus, extends 

 throughout the entire length of the cord. 



Structure. The columns of the cord are directly continuous with those of the 

 medulla oblongata, below ; but, higher up, both the white and gray constituents 

 are re-arranged before they are continued upwards to the cerebrum and cere- 

 bellum. 



The anterior pyramid is composed of fibres derived from the anterior column 

 of the cord of its own side, and from the lateral column of the opposite half of 

 the cord, and is continued upwards into the cerebrum and cerebellum. The 

 cerebellar fibres form a superficial and deep layer, which pass beneath the olive 

 to the restiform body, and spread out into the structure of the cerebellum. A 

 deeper fasciculus incloses the olivary body, and, receiving fibres from it, enters 



the pons as the olivary fasciculus or 

 Fig. 262. Transverse Section of Medulla rm i i_ 1.^.1 i & P/>T_ r 



Obiouo-ata. fillet; but the chiet mass oi iibres trom 



the pyramid, the cerebral fibres, enter 

 the pons in their passage upwards to 

 the cerebrum. The anterior pyramids 

 contain no gray matter. 



The lateral tract is continuous, be- 

 low, with the lateral column of the cord. 

 Its fibres pass in three different direc- 

 tions. The most external join the 



restiform body, and pass to the cerebellum. The internal, more numerous, pass 

 forwards, pushing aside the fibres of the anterior column, and form part of the 

 opposite anterior pyramid. The middle fibres ascend, beneath the olivary body, 

 to the cerebrum, passing along the back of the pons, and form, together with 

 fibres from the restiform body, the fasciculi teretes, in the floor of the fourth 

 ventricle. 



Olivary body. If a transverse section is made through either olivary body, it 

 will be found to consist of a small ganglionic mass, deeply imbedded in the medulla, 



raterur ffaare 



ieuli Ttretel 



