MOKPHOLOGT OF THE BEAIX IX THE MAMMALIA. 333 



rurrows unitiua: at an aiiLi'Ic of 1:^0 . The vetrocalcarine sulcus is coufined to the 

 cerebellar area. It passes backward wiili a very slight upward inclination for a distance 

 of S nun. The degree of ()bli(iuity of this sulcus shows considerable variability. In 

 some eases it is almost horizontal ; in otliers it is removed as much as 30" from that 

 plane. At its meeting place Avitb tlie ealcarine sulcus it is as deep as the latter. 



The paraealcariue sulcus ]);isscs obliquely upward and forward from the angle of 

 junction of the otlier two sulci almost as far as (in rare cases even actually crossing) the 

 dorsal margin. In some cases its terminal piece is bent vertically upward. This sulcus 

 is generally shallower than the ealcarine, so that it appears to cut deeply into the upper 

 lips of (without actually joining) the sulcu.s formed by the junction of the ealcarine and 

 the retrocalcarine furrows. In one ease, however, I have seen the ealcarine and para- 

 ealcariue sulci joined, the retrocalcarine being the separate element. This is instructive, 

 because it shoAvsthat the reti-ocalcarine sulcus possesses a certain amount of individuality 

 and is not a mere mechanically-produced extension of the ealcarine sulcus. The sulcus 

 b varies considerably in length and depth in different brains. In the large brain of a 

 Lemur rariiis I have seen it represented by a veiy faintly marked depression less than 

 3 mm. long. In other cases it is a furrow 8 mm. long and 2 or even 3 mm. deejD. It 

 undoubtedly represents the most caudal element of that composite sulcus of the human 

 bi-ain known as the " collateral." 



The sulcus c is even more variable. It may be a mere pit in the depression caused by 

 the roof of the bony capsule of the floccular appendage (so-called " petrosal " lobule) of 

 the cerebellum, or it may become a deep sulcus 5 or 6 mm. long. 



On the external aspect of the neopallium the sulci are no less variable (tigs. G & 7). 

 The deepest and most constant furrow is that which is commonly called the Sylvian 

 fissure. It really consists of an apparent blending of two actually separate overlapjjing 

 sulci. One of these, which I shall call pseudosylvian, proceeds vei'tically upward (for a 

 distance of 4 mm.) from the bend (or more strictly from the posterior limb) of the rhinal 

 fissure. It ap2)e;u's to (but does not actually) join a second sulcus, which is obliquely 

 placed, so that the two sulci form an angle of abuut 150'. This upper oblique sulcus I 

 shall call the " suprasylviau." 



If the lips of these sulci be separated, it will be found that the sulcus pseudosylvius 

 cuts deeply backward into the hemisphere, so that its posterior lip forms an operculum 

 (I mm. long and almost equally thick) which overlaps a dejjressed area (fig. 6 a). The 

 lower part of the sulcus suprasylvius overlaps the s. pseudosylvius (fig. (5), and the anterior 

 lip of this part of the furrow is also slightly operculated. Hence there is an area 

 (about 6 mm. long and 1 mm. high) submerged and hidden I'rom view by the anterior 

 opercular lip of the supi'asylvian and the posterior opercular lip of tiie pseudosylvian 

 sulci (fig. 6 a). The few observers who have in any way referred to this depressed area 

 regard it as the insula Keilii. IIoll {op. cit. supra) regards it as the submerged 

 homologue of Leuret's first arcuate gyrus of the Carnivora. The degree of accuracy of 

 these statements will be discussed below. 



Between the upper end of the suprasylviau sulcus and the dorso-mesial edge of the 

 hemisphere there is a sagittal sulcus of very variable length, wliich may be distinguished 



