364 DR. G. ELLIOT SMITH ON 



4'5 mm. on each side of the mesial plane, and then bends obliquely forward and ontward 

 to reach the anterior surface of the organ. In this way it cuts off a lozenge-shaped area 

 on the dorsal surface, which forms part of the anterior lobe. On the anterior surface the 

 fissure extends obliquely downward and \^'ith a slight lateral inclination, and ultimately 

 reaches the situation where the middle peduncle dips into the cerebellum. 



By means of this deep cleft a lozenge-shaped area on the dorsal surface, a large central 

 semicircular area on the cephalic surface, and a long caudally-extending worm-like 

 intercolumnar process on the ventral surface are completely separated from the rest of 

 the cerebellum to form the lob/is imticMs. Stroud, following the precepts of Burt Wilder 

 as regards nomenclature, calls this lobe by the mononym "■ preramus.'" Kuithan calls it 

 " Vorderlappen" Thus both Stroud and Kuithan, on embryological grounds, support 

 the suggestion, which I am advancing for comparative reasons, to separate the region in 

 front of Xhefisswa prima from the rest of the cerebellum as a natural primary division, 

 which we may appropriately distinguish as the lobus anticus. 



The part of the cerebellum which is left after removing the lobus anticus and the two 

 lobi Jlocculi is the most complex part of the organ, and, as its subdivisions are not so 

 clearly defined as those of the other lobes, opinions are divided as to tlie most natural 

 method of subdivision. We may at once neglect Kuithan's subdivision, because he 

 fails to recognize the ujiper part of the parajlocculus in the Sheep, but simply calls it 

 vermis lateralis, without suggesting any homology. Otherwise he says little about the 

 xe^\orihQ\\m(\.t\\efissnra prima. Stroud calls the whole of this area " postramus." He 

 calls each of the lateral areas of the "jwstramns " a ^j^'fcww?, and divides each into a 

 " prep ileum " and a " postp) ileum." 



The objections to the acceptance of tliis suggestion of Stroud are twofold. In the first 

 place, this mode of dividing up the lateral areas is not sufficiently constant in the lowlier 

 mammals to be fundamental, and if it were it is not sufficiently comprehensive ; in other 

 words, this lateral area falls more naturally into three (or perhaps four) subdivisions. In 

 the second place, tliere is, in the area behind the fissura prima, a fissure of higher mor- 

 phological importance and of greater stability than Stroud's " interpileur sulcus." This 

 fissure I shall call i\\e fissura secunda, because, of the fissures that cross the mesial plane, 

 it is second in importance only to the fissura jirima. The fissura secuuda corresponds to 

 the cleft which separates the pyramid from the uvula in the human brain. Stroud calls 

 it the Mt'wiar SMfc?fs, and demonstrates its early appearance in the Cat, for be confesses 

 (p. 106) that be does not know wbethcr it or tlie fissura prima develops the earlier. 

 Kuithan's figures show the precocious appearance of the fissura secunda soon after the 

 ■fissura prima in the Sheep. But my main reason for adopting this fissure as an interlobar 

 boundary is a comparative one. In all the various types of cerebellum which form a 

 complete gradation from that of Notoryctes to that of Man, this fissure is constant, and 

 separates a small caudal area of cerebellum from the complex central mass. Its mor- 

 phological importance is undoubtedly considerably inferior to that of the fissura prima, 

 bvit to the student of the lowlier mammalian tvpes the adoption of ihe fissura secuuda as 

 an interlobar fissure will facilitate descriptive work very considerably. 



I propose, therefore, to divide the mesial or " interfioccular '" area of the cerebellum 



