348 NERVOUS SYSTEM. 



increase has affected principally the surface of the roof, so that 

 as the cerebellum has grown it has been thrown into many 

 folds, the exact form cf which varies in different specimens. 

 The cerebellum has at the same time increased in size and has 

 thus extended laterad as well as caudad and craniad. It thus 

 touches the cerebrum in front (separated from it by the ten- 

 torium) and aids it in concealing the midbrain and 'tween-brain 

 in dorsal view, while caudad in the same view it conceals the 

 greater part of the medulla. The connections of the cerebellum 

 with adjacent parts of the brain are also overhung and con- 

 cealed. 



The whole surface of the cerebellum is thrown up into 

 numerous folds or gyri, separated from one another by deep 

 fissures or sulci, which appear at first to render the surface 

 wholly irregular. The entire mass is, however, divisible into 

 a central portion, which from its resemblance to a segmented 

 worm is called the vermis (/) (its cranial part is the superior 

 vermis, and its caudal part the inferior vermis), and into lateral 

 portions, the hemispheres (/). The vermis (/) occupies a 

 median longitudinal position, and its gyri and sulci are in the 

 main transverse. It is not directly connected with adjacent 

 parts, and its ventral surface extends farther caudad and craniad 

 than that of the hemispheres. The ventral part of the superior 

 vermis is fitted against the posterior corpora quadrigemina. 



The hemispheres (k] may again be subdivided into groups 

 of gyri which have received special names. One of these, the 

 so-called appendicular lobe, fits into the appendicular fossa of 

 the petrous bone. 



The cerebellum is connected to the adjacent parts by three 

 tracts of fibres, sometimes known as crura cerebelli. The tract 

 connecting it with the medulla oblongata is the corpus resti- 

 forme ; that connecting it with the pons is the brachium 

 pontis (Fig. 141, /) ; these have been described. A third 

 tract passes craniad to the corpora quadrigemina (Fig. 141, p 

 and q); this is the brachium conjunctivum (Fig. 141, k). 



The cerebellum is composed of white and gray matter, the 

 latter on the surface (Fig. 143, ///). The folds of its surface 

 present thus a contrivance for increasing the amount of gray 



