THE CEREBELLUM 777 



are separated from the vermis by two deep paramedian fissures. They he in the 

 lateral depressions of the cerebellar compartment of the cranium. 



In tracing the fissures from behind forward it will be noticed that they are nearly sagittal 

 as far as the anterior sui-face, where they diverge widely, so that the vermis forms all of the fore 

 part of the cerebellum. 



The surface of the cerebellum is further cut up into numerous gyri (cerebelli) 

 by narrow and relatively deep sulci (cerebelli), many of which approach a trans- 

 verse direction. Certain of the sulci are more pronounced than the others, and by 

 means of them it is possible to define groups of gyri. Such groups are termed 

 lobes, and have received specific names, derived chiefly from the systematic de- 

 scriptions of the human cerebellum. 



The lobes of the vermis are readily distinguished on median section. Enumerated from 

 the anterior to the posterior extremity they are: (1) lingula, (2) lobus centralis, (3) lobus ascendens, 

 (4) lobus culminis, (5) lobus clivi, (6) tuber vermis, (7) pyramis, (8) uvula, (9) nodulus. Each 

 hemisphere is cut into laterally by two sulci which mark off two sagittal discoid masses, termed by 

 Ziehen tabulations. The lateral tabulation consists of four or five lobules, the lowest of which 

 is regarded as the flocculus. The medial part of the hemisphere is divided into three or four lobes. 

 Martin proposes the term tractus for the sagittal masses; on this basis the vermis would become 

 the tractus medianus, and the others tractus laterales (primus, secundus, etc.). In the absence 

 of a satisfactory morphological basis it seems undesirable to deal with the lobation of the cere- 

 bellum in fiirther detail. 



The cerebellar peduncles (Pedunculi cerebelli), three on each side, join the 

 central white matter of the cerebellum at the base. The posterior peduncle is the 

 restiform body of the medulla, a large rounded tract derived from the lateral and 

 ventral columns of the cord. Near the middle of the medulla it inclines outward, 

 forms the lateral wall of the fourth ventricle, and ends by entering the central white 

 matter of the cerebellum. The middle peduncle is formed, as previously seen, by 

 the brachium pontis. The anterior peduncles (Brachia conjunctiva) pass forward 

 on either side on the dorsal surface of the pons, forming the lateral boundary of the 

 fore part of the fourth ventricle. They disappear under the corpora quadrigemina 

 into the substance of the mid-brain. At the point of disappearance the trochlear 

 (fourth) nerve emerges from the mid-brain. In some cases two or three bundles of 

 fibers (Fila lateralia pontis) arise in the angle between the middle and anterior' 

 peduncle, curve obliquely forward and downward over the outer aspect of the 

 latter, and spread out on the ventral face of the cerebral peduncle just in front of 

 the pons. 



On sagittal section the cerebellum is seen to consist of a layer of gray cortical 

 substance (Substantia corticalis) and the white medullary substance. The white 

 matter consists of a large basal mass (Corpus medullare), which is joined by the 

 peduncles and gives off primary laminae (Laminae meduUares) to the lobules; 

 from these secondary and tertiary laminae arise, the latter entering the gyri. The 

 arrangement on sagittal section is tree-like, hence the term "arbor meduUaris," 

 which is applied to it.'- The central gray matter consists of groups of cells which 

 form small nuclei embedded in the central white substance. 



The principal connections established by the peduncular fibers of the cerebellum are as 

 follows: The posterior peduncle (Corpus restiforme) is composed of afferent and efferent fibers 

 which connect the cerebellum with the medulla and spinal cord. The cerebello-spinal fascicu- 

 lus or direct cerebellar tract, which arises from the cells of the nucleus dorsalis (Clarke's column) 

 of the cord, ends in the cortex of the vermis; many of its fibers cross to the opposite side. Nu- 

 merous arcuate fibers from the nucleus gracilis and nucleus cuneatus of the same and opposite 

 sides estabhsh connections with cells of the cerebellar cortex. Olivo-cerebellar fibers (chiefly 

 afferent) connect with the ohvary nucleus of the same and of the opposite side of the medulla 

 oblongata. The nucleo-cerebeUar fasciculus comprises fibers derived from the nuclei of the 

 fifth, eighth, and tenth cranial nerves (Edinger). The descending cerebello-spinal fasciculus 

 consists of fibers which terminate in relation with cells of the ventral horns of the spinal cord. 

 The chief facts concerning the middle peduncle have been mentioned in the description of the 

 pons. The anterior peduncle is essentially an efferent tract, the fibers of which pass forward 



' Also known as the "arbor vitae." 



