248 A MANUAL OF ANATOMY. 



foramen magnum, below which it is continued as the spinal 

 cord. It is about three-quarters of an inch wide at the 

 pons and three-eighths of an inch at its terminus. 



The Under Surface of the Medulla. Through the 

 middle of the medulla from before backward extends a 

 fissure the anterior median fissure which divides it into 

 two symmetrical halves. The fissure terminates at the pos- 

 terior margin of the pons in a slightly enlarged cavity 

 named the foramen caecum. 



About an inch behind the pons the anterior fissure will 

 be found crossed by fibres passing from one side of the 

 medulla to the other. This is the decussation of the an- 

 terior pyramids of the medulla. 



Along the side of the anterior median fissure lie two well- 

 marked ridges or bundles, which are the anterior pyramids 

 of the medulla. They are limited externally by shallow 

 grooves termed the anterolateral sulci; and external to 

 these grooves are the lateral tracts or columns of the 

 medulla. 



At the outer side of the anterior pyramids and close to 

 the pons is developed a small oval nucleus the olivary 

 body. The anterolateral groove is continued along the 

 front of the olivary body and an offset from it around the 

 back part called the posterior olivary sulcus. 



Just at the olivary body the anterolateral groove is in- 

 terrupted by a thin lamella of fibres which curve around 

 from the anterior to the posterior parts of the nucleus and 

 are called the external arciform fibres. 



DISSECTION. 



Divide the cerebellum along its median plane, open the halves, and expose 

 the fourth ventricle. 



