286 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



found it on measurement to be ^ of an inch. In one of five months it was 

 about the same. Between the fifth and sixth month it gradually becomes narrower. 

 In a foetus of the middle of the sixth month it measured ^ of an inch. Its length 

 also increases as development advances. At three months the fossa was i of an inch 

 long, at three and a half months, I of an inch ; at four and a half months, f of an 

 inch ; at five and a half months, 1 of an inch. Its general shape also changes. Thus, 

 at the end of the third month it has the appearance of an oval-shaped fossa. This 

 oval shape is still present at the middle of the fourth month, Plate XXXV, fig. 3. 

 At the middle of the fifth month this fossa has assumed a triangular shape, Plate 

 XXXV, fig. 5. This triangular shape is also preserved at the middle of the sixth 

 month, Plate XXXV, fig. 7. In both of these the floor is seen raised up. This is 

 the portion that has received the name of the central lobule or Island of Reil. 

 Ecker, in a brief note on the development of this fissure, does not give the shape 

 as triangular at this early period. He says, " at the end of the sixth month this 

 oval trench is changed by the formation of the anterior or upright branch (fissura 

 Sylvii) into a triangular one." (Edes' transl.) The figures of PI. XXXV will 

 show, however, that the shape of this fossa is triangular some time before the ante-, 

 rior branch of the Sylvian makes its appearance, there being no indication of its 

 presence in either figure 5 or 7. This wide fossa is gradually narrowed. The tem- 

 poral lobe grows upward to some extent, but the closing is mainly effected by the 

 development downward of the frontal. This development is rather peculiar. It is 

 not by a direct growth downward of the whole lobe but by a fold of the frontal 

 rolling downward and backward over the floor of the fossa, gradually shelving it 

 in. In Plate XXXV, fig. 5, the formation of this shelf is just perceptible, while 

 in fig. 7 it may be seen extending downward to a slight extent, the thickness of 

 the fold being at this stage about one-eighth of an inch. In this way, as may be 

 seen by comparing figs. 3, 5 and ~ of Plate XXXV, and figs. 1, 3 and 7, 

 Plate XXXVI, is the fossa gradually filled up, until it becomes, at about the time 

 of birth, a mere fissure. In this way is the Island of Reil gradually covered. The 

 fold has, from this, received the name of the operculum, because it covers or con- 

 ceals the central lobe. The fissure of Sylvius thus at an early date divides, an- 

 teriorly, the hemisphere into two lobes. 



At the end of foetal life the fissure is not yet completely closed and the Island 

 of Reil may be distinctly seen without pulling apart the two edges of the fissure. 

 In the adult white brain this concealed lobe is not visible. Curiously enough, how- 

 ever, in the brain of the negro it is distinctly visible; the brain of the negro approach- 

 ing in this respect a foetal character. I have observed this condition in many negro 

 brains that I have examined with special reference to this point. Marshall also 

 found it in the brain of a Bushwoman, and Gratiolet described the same condition 

 as existing in the brain of the Hottentot Venus. Both of these writers recognized its 

 foetal character, and it would, therefore, seem with good reason to be a characteristic 

 of the negro brain as compared with that of the white race. An interesting fact in 

 this connection is that in the brain of several mulattos the Island was also visible but to 



