II04 



HUMAN ANATOMY. 



fold, the plica chorioidea, bounds a narrow recess that extends along the inferior surface of the 

 cerebellar lamina. This recess is only temporary and is soon obliterated by the subsequent at- 

 tachment of the roof- membrane to the inferior surface of the cerebellar lamina. The succeeding 

 stage (Fig. 955, C) emphasizes the alteration in the planes of the cerebellar surfaces, the former 

 superior now becoming the anterior, the anterior the inferior, and the inferior the posterior. 

 From the posterior margin of the dorsal surface the choroid fold dips into the brain-cavity. 

 Between the mid-brain and the cerebellum now stretches the first definite indication of the later 

 superior medullary velum. In agreement with His, Bolk recognizes that the former intraven- 

 tricular (inferior) surface has now become an extraventricular one and that the permanent attach- 

 ment of the plica chorioidea corresponds to a secondary and not to the primary line of union. 

 The stage represented in Fig. 955, D is important, since it marks the beginning of the first 

 fissures. One of these, the sulcus pri)Harius (the Jisstira priina of Elliot Smith), appears as a 

 transverse groove on the upper part of the anterior surface and thus early establishes the funda- 

 mental division of the cerebellum into an anterior and a posterior lobe. The other fissure 

 appears in the median area near the posterior margin of the cerebellum and is the sulcus post- 

 nodularis. On each side (Fig. 956, A ) an additional fissure cuts off a narrow tract that embraces 

 the postero-lateral area of the cerebellum. This fissure, \.\\e sulcus floccularis, for a time remains 

 ununited with the postnodular sulcus ; but later, with its fellow, it becomes continuous with 

 the postnodular sulcus and thus defines a narrow band-like tract, the median part of which 



Fig. 956. 



Six stages in development of human cerebellum, from fcetuses of 9 (A), 13 (B), 15 (C), 22 (Z)), 25 (.£), and 32 cm. 

 (F) length; /, sulcus primarius (preclival) ; 2, s. floccularis ; 3, s. postnodularis ; 4, s. infrapyramidalis ; 5, s. 

 superior posterior (postclival) ; h. great horizontal fissure; mb, mid-brain; r, roof-membrane; Ir, lateral recess: 

 w, nodulus; m, uvula;/, pyramis ; <, tuber ;y, folum. (^Drawn from figures of Bolk.) 



eventually becomes the nodule, the lateral portions the flocculi, whilst the intervening strips 

 become the floccular peduncles and part of the inferior medullary velum. The diverticulum 

 bounded on each side by the floccular area is the beginning of the lateral recess of the fourth 

 ventricle and is early filled by the rapidly growing choroid plexus. A shallow transverse 

 groove, the incisura fasligii, just suggested in Fig. 955,(7 but distinct in the succeeding sketch, 

 marks the beginning of the tent-like recess that later conspicuously models the roof of the 

 fourth ventricle. Coincidently with and about midway between the fissures just described, a 

 third furrow appears on the posterior cerebellar lobe. This is thejissura secunda (Elliot Smith) 

 or the infrapyramidal sulcus. Very shortly a fourth groove appears behind the sulcus primarius 

 and marks the beginning of the prepyramidal fissure. In this manner the median tract of the 

 posterior lobe 's early subdivided by three fissures into four areas, which, from behind toward 

 the sulcus primarius, give rise to the nodule, the uvula, the pyramid and a still undifferentiated 

 zone. By the subsecjuent appearance of additional furrows, this narrow zone gives origin to the 

 tuber, the folium cacuminis and the clivus. Meanwhile on the anterior lobe of the cerebellum 

 three short transverse fissures appear, by which the anterior end of the worm-tract is broken 

 up into areas that, while establishing subdivisions of morphological value (Bolk). are later lost 

 in the uncertain foliation of the lingula and lobulus centralis of the mature cerebellum. 



After the fundamental subdivision of the median area (worm) has been accomplished, the 

 lateral mas.ses (hemispheres) of the cerebellum become '=ubdivided into definite tracts (lobules) 

 by fissures that appear during the fourth and fifth months of foetal life. The lateral extensions 



