582 NERVOUS SYSTEM. 



the corresponding restiform body. The upper part of the posterior pyramids 

 forms the lateral boundaries of the calamus scriptorius. 



The posterior surface of the medulla oblongata 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 diver- 

 gence of the posterior pyramids and restiform bodies, opens to view the gray 

 matter of the medulla, which is continuous, 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, terminating, below, at the point of the 

 ventricle, 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 rearranged before they are continued upwards to the cerebrum and cere- 

 bellum. 



The anterior pyramid \$ 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 

 olivary body 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 fillet; but the chief mass 

 of fibres from 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, below, with the lateral column of the cord. 

 Its fibres pass in three different directions. The most external join the resti- 

 form 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 

 Fig. 326 -Transverse Section of teretes. in the floor of the fourth ventricle. 



Olivar y boj y- If a transverse section is made 

 through either olivary body, it will be found to 

 consist of a small ganglionic mass, deeply im- 

 bedded in the medulla, partly appearing on the 

 surface as a smooth, olive-shaped eminence (Fig. 

 326). It consists, externally, of white substance; 

 a nd, internally, of a gray nucleus, the corpus 

 dentatum. The gray matter is arranged in the 

 form of a hollow capsule, open at its upper and inner part, and presenting a 

 zigzag, or dentated outline. White fibres pass into, or from the interior of 

 this body, by the aperture in the posterior part of the capsule. They join with 

 those fibres of the anterior column which ascend on the outer side, and beneath 

 the olivary body, to form the olivary fasciculus, which ascends to the cerebrum. 

 The ri'stlforiii hody is formed chiefly of fibres from the posterior column of 

 the cord; but it receives some from the lateral column, and a fasciculus from 

 the anterior, and is continued, upwards, to the cerebrum and cerebellum. On 

 entering the pons, it divides into two fasciculi, above the point of the fourth 

 ventricle. The external fasciculus enters the cerebellum: the inner fasciculus 

 joins the posterior pyramid, is continued up along the fourth ventricle, and is 

 traced up to the cerebrum with the fasciculi teretes. 



Septum of the medulla oblongata. Above the decussation of the anterior pyra- 

 mids, numerous white fibres extend, from behind forwards, in the median line, 

 forming a septum, which subdivides the medulla into two lateral halves. Some 



