io82 



HUMAN ANATOMY. 



locus cceruleus to this part of the ventricle (page logs). Mesial to these cells th^ posterior longi- 

 tudinal fasciculus shows, in transverse section, as a triangular field close to and on each side 

 of the raphe. 



The most conspicuous feature of the dorsal part of the section is the comma-shaped fibre- 

 tract of the superior cerebellar peduncle (brachium conjunctivum). The thicker part of the tract 

 lies dorsally and its thinner edge cuts into the lateral part of the posterior area of the pons 

 about half way between its dorsal and ventral boundaries. Between the cerebellar tract and the 

 lateral angle of the ventricle, a slender crescentic strand of transversely cut fibres marks the 

 descending motor or mesencephalic root of the trigeminal nerve. The tract of the median fillet 

 no longer touches the raphe, but lies as a compressed and horizontally elongated oval along the 

 ventral border of the dorsal field. The three-cornered area included between the outer end of 

 the mesial fillet, the cerebellar arm and the surface, contains a curved triangular tract that 

 sweeps backward and insinuates its pointed dorsal extremity along the outer side of the cere- 

 bellar strand. This tract is the lateral fillet (lemniscus lateralis), an important part of the 

 pathway by which auditory impulses are carried from the reception-nuclei of the eighth nerve 

 to the inferior corpora quadrigemina, the internal geniculate body and the cerebral cortex. A 

 collection of small nerve-cells, embedded within the outer angle of this tract, gives rise to a 

 number of its component fibres and is, therefore, known as the nucleus of the lateral fillet 

 (nucleus lemniscus lateralis). An additional group, between the lateral fillet and the cerebellar 

 tract, constitutes the nucleus tegmenti lateralis (Kolliker). The remainder of the tegmental 

 area is occupied by the formatio reticularis. 



THE CEREBELLUM. 



The cerebellum — the "little brain," in contrast to the cerebrum or "great 

 brain" — is placed in the posterior fossa of the skull and beneath the tent-like shelf 

 of dura, the tentorium, which separates it from the overlying posterior part of the 



Fig. 937. 



Pons 



Anterior crescentic lobule 



Great horizontal 

 fissure 



Postero-superior 

 lobule 



Postero-inferior 

 lobule 



Middle cerebellar 

 peduncle 



Medulla 



Accessory flocculus 



Flocculus 

 Biventral lobule 



Tonsil 



Pyramid Posterior cerebellar notch Tuber 



Cerebellum viewed from in front and below; pons and medulla occupy greater part of vallecula and mask worm. 



cerebral hemispheres. It lies behind the pons and medulla and the fourth ventricle, 

 with the roof of which space it is intimately related. By means of its three peduncles 

 — inferior, middle and superior — the cerebellum is connected with the medulla, the 

 pons and the mid-brain respectively. 



The general form of the cerebellum is that of an ellipsoid, compressed from 

 above downwi^rd and constricted, save on the dorsal aspect, by a median groove of 

 varying proportions. Its greatest dimension is the transverse diameter, about 10 cm. 

 (4 in. ) ; its least is the vertical (3 cm. ), while in the sagittal direction the cerebellum 

 measures about 4 cm. in the mid-line and about 6 cm. at the side. The cerebellum 

 weighs about 140 gm. (5 oz. ) and constitutes approximately one-tenth of the entire 

 brain-weight. 



The conventional division into a narrow median part, the worm, and the two 

 lateral expansions, the hemispheres, while convenient for the description of the 

 cerebellum of man, is not warranted by recent comparative and developmental 



