THE CEREBELLUM. 71 



The greatest diameter of the organ is transverse, and extends to about four inches 

 (10 centimeters) : its greatest width from before backwards is about two inches (5 

 centimeters) : of the middle part about one inch and a half (4 centimeters) ; and 

 its greatest depth is about two inches, but it thins out towards the lateral border. 

 It weighs about 5 oz. 



The cerebellum is characterised by its laminated or foliated appearance, its 

 surface being everywhere marked by deep, closely set, transverse and somewhat 

 curved, fissures, which extend a considerable depth into its substance, but do not all 

 entirely encircle the organ, for many of them coalesce with one another, and some 

 of the smaller furrows have even an oblique course between the others. Moreover, 

 on opening the larger fissures, other folia are seen to lie concealed within them, not 

 reaching the surface of the cerebellum. 



The depth of the fissures can best be estimated in sections through the organ 

 taken across the laminae : in such sections each lamina is seen to have a white 

 centre and a grey cortex, and the white centres of the laminee appear in the form of 

 processes ramifying from a larger white centre near the middle and anterior part of 

 the organ : to the arborescent appearance thus obtained in section the name arbor 

 vitce, cerelelli has been applied. 



Externally the most conspicuous fissure is the great horizontal fissure (figs. 

 56, 58, fh), which beginning in front at the middle peduncle of either side 

 extends round the outer and posterior border of each hemisphere, dipping down into 

 the posterior notch. This fissure divides the cerebellum into an upper and lower 

 portion, corresponding in fact to the upper and lower surfaces, in each of which 

 several lobes, separated by fissures for the most part deeper than the rest, are 

 described. Taking the great horizontal fissure to divide the cerebellum into an 

 upper and a lower half or surface, we may first consider the parts which are present 

 upon the upper surface and which .compose the upper half of the organ in both the 

 worm and the hemispheres. 



UPPEE SURFACE. The upper worm begins at the superior medullary velum 

 between the two superior peduncles and ends at the bottom of the posterior notch in 

 a short, concealed transverse lamina, termed the folium cacuminis. It is usually 

 described as being formed of five successive parts or lobules, which are termed from 

 before back the lingula, the central lobe, the culmen (culmen monticuli), the clivus 

 (clivus monticuli), and the folium cacuminis. Corresponding with these in each 

 hemisphere are the large poster -o- superior lobe, continued laterally from the small 

 folium cacuminis, the posterior crescentic, continued from the clivus, the anterior 

 crescentic lobe, continued laterally from the culmen, the ala loibuli centralis, continued 

 from the central lobe, and sometimes a lateral extension of the lingula. 



Of the fissures which separate these lobes of the upper surface from one another, 

 four in number, the first, or most anterior, which may be termed the precentral 

 sulcus, lies in front of the central lobe, and separates it from the lingula. The 

 next, or postcentral sulcus, divides the central lobe and its alas from the culmen 

 and anterior crescentic lobes. The third, or preclival fissure (also called the antero- 

 superior), separates the culmen and anterior crescentic from the clivus and posterior 

 crescentic lobes. Lastly, the fourth, or postclival fissure (also called the postero- 

 superior), lies immediately over the folium cacumiuis, and separates this and the 

 postero-superior lobes from the clivus and posterior crescentic. Below the folium 

 cacuminis and the postero-superior lobes comes the great horizontal fissure which 

 has been already described. 



Besides these interlobar fissures, all of which extend deeply down towards the 

 central white substance, there are certain other sulci which on the upper surface 

 of the hemispheres are almost or quite as well marked as the interlobar ones, but 

 which are less deep or are not seen on the upper worm. They are best made out in 



