THE CEEEBELLUM. 



571 



Cerebellar rudiment 



Tsenia 



The accentuation of the pontine flexure at this stage brings the two cerebellar 

 idiments into the transverse direction and in line one with the other and with 

 le roof-plate, which is now being thickened by immigrant neuroblasts from the 

 icdial extremities of the two cerebellar rudiments. When one organ is thus 

 rmed by the union in the roof-plate of the originally separate rudiments, it 

 resents the form of a dumb-bell shaped mass (Fig. 503). Upon the inferior 

 ;pect of this mass there is a slight ridge, to 

 hich the tela chorioidea ventriculi quart! is 

 Cached. Opposite the lateral cerebellar rudi- 

 ients (but not in the median plane) the attach - 

 ient of the tela becomes thickened to form the 

 Dsterior medullary velum. 



Early in the third month the growth of the 

 >rebellar rudiment begins to manifest itself by 

 teral bulgings of its surface. 



The rhombic lip, the inferior part of which 

 is been seen to play an important part in the 

 ivelopment of the nuclei pontis and nucleus 

 ivaris inferior, is also continued upwards beyond 

 le pontine flexure on to the cerebellar rudiment, Fia - 503. DORSAL ASPECT OF THE RHOMB- 

 here it forms a marginal fringe. Thus/even 



L the second month, a groove can be detected upon the cerebellum separating off 

 band which is continuous with the tuberculum acusticum. The part nearest to 

 le tuberculum represents the rudiment of the flocculus and the medial extremity the 

 )dulus (Fig. 503). During the third month the cerebellum appears as a rounded 

 ir transversely placed across the upper part of the roof of the fourth ventricle, 

 id as the lateral extremities of this bar expand (Fig. 504), it assumes a dumb-bell 

 tape not unlike that presented a few weeks earlier (Fig. 503) on its ventricular 

 ,pect. As these lateral bosses (lobi laterales) develop, a mass of transverse fibres 



connexion with them also becomes apparent. It represents the fibres trans- 

 srsae of the pons. They arise from the cells (nuclei pontis) which have wandered 

 to the basal lamina of the metencephalon from the rhombic lip of the myelen- 

 phalon (Fig. 499) ; and the fibres which enter each cerebellar boss come mainly 

 om the nuclei pontis of the other side. Towards the end of the fourth month, 

 I 1 even a month earlier in some cases, a little bud grows out from the cerebellum 

 L each side immediately above the flocculus. It is the paraflocculus or flocculus 



secundarius. In man it never attains 



Fiss.secunda. 



Fiss. suprapyramidalis. 



' Fiss. prima. / Lobus 



Velum 



medullare 

 posl-erius./ 



lareralis. 



^Parafloc. 



Floe. 



Recessus 



lareralis 



v venh. quarH. 



Tuberculum acusHcum. 



Medulla oblongafa. 



Modulus. Obex. Taenia venlriculi quarH. 



a large size, but in most mammals it 

 develops into a large lobe, even as big 

 as one -third the size of a cerebellar 

 hemisphere (in the manatee), and in 

 many animals a deep fossa is formed in 

 the temporal bone to lodge this part of 

 the cerebellum. 



As the cerebellum grows the lateral 

 hemispheres expand much more rapidly 

 than the median part the handle of the 

 dumb-bell. But the superficial area 



J. 504. THE POSTERIOR ASPECT OF A FCETAL r. ,-, i , -, . - -, -, 



(FOURTH MONTH) CEREBELLUM, MEDULLA OB- of the latter becomes increased by means 

 LONGATA AND FOSSA RnoMBoiDEA. of transverse folds which begin to make 



their appearance at the close of the 



ird month. Earlier in that month the median part of the cerebellum presents 



sagittal section almost a semicircular outline (Fig. 50*7, A) with a slight notch 



its inferior margin (fissura postnodularis) demarcating the nodulus. As 



ilopment proceeds during the third month the nodular region becomes bent 



wards upon the rest of the cerebellum (Fig. 507, B), thus starting the posterior 



< rertieulum of the fourth ventricle, which ultimately assumes a . tent-like outline 



ig- 519). 



At the close of the third month the irregular growth of the surface of the 



