SPECIAL ANATOMY. 159 



iter a tertio ad quarhim ventriculum with the third ventricle. 

 Its posterior part, frora its likeness to the nib of a pen, is 

 termed calamus scriptorius, and is continued through the 

 spinal cord by a canal which extends centrally between its 

 columns. The cerebellum is a round body, situated in a 

 special division of the cranium, posteriorly to the cerebrum 

 from which it is separated by the ossific and the mem- 

 branous tentoria ; and to which it is connected on each side 

 by the iter e cerebello ad testes. It presents, on either 

 side of the median line, running from behind forwards a 

 fissure, and thus the organ is divided into three parts. 

 Centrally is a " caterpillar-shaped " body, the anterior part 

 of which is the anterior vermiform process; the posterior 

 part, the posterior vermiform process, on either side of this 

 is a lobe. By an incision from behind forward in the median 

 plane we may expose the internal structure of the organ. 

 The surface of the cerebellum presents convolutions which 

 are divided into uniform parts by transverse secondary 

 grooves. The grey matter which the external surface 

 presents, is arranged with regard to the white as the leaves 

 of a tree are to the branches, so the white matter inferiorly 

 terminates in abroad base. This arrangement is the arhor 

 vitce, and the base of the stem is formed by three tracts of 

 white matter, the crura of the cerebellum. The anterior 

 is the iter e cerebello ad testis, the posterior is continuous 

 with the corpus restiforme of the medulla oblongata, the 

 middle passes under the medulla oblongata, joining with 

 its fellow to form a transverse band at the base of the 

 brain, the pons Varolii; this is the part which serves 

 to connect together the three other divisions of the brain. 

 It presents a longitudinal groove along its central line, 

 through which the basilar artery passes, and on either 

 side of which it becomes more prominent, for here is a 

 collection of nerve-cells in its substance constituting the 

 corpus dentatum, the special centre of the fifth pair of 

 nerves which arises from the lateral parts of its under 

 surface. 



The medulla oblongata is but the commencement por- 

 tion of the spinal cord, from which it is distinguished by 

 its greater size, caused by the presence of a few extra nerve 

 cells collected to form the corpus olivare. In addition to 

 this, like the spinal cord, it presents three tracts of white 

 matter (externally) continuous with those of the cord 



